Vox Charta » Instrumentation and Methods

Data analysis recipes: Probability calculus for inference [Cross-Listing]

astro-ph — Tue, 22 May 2012 00:46:12 +0000

In this pedagogical text aimed at those wanting to start thinking about or brush up on probabilistic inference, I review the rules by which probability distribution functions can (and cannot) be combined. I connect these rules to the operations performed in probabilistic data analysis. Dimensional analysis is emphasized as a valuable tool for helping to construct non-wrong probabilistic statements. The applications of probability calculus in constructing likelihoods, marginalized likelihoods, posterior probabilities, and posterior predictions are all discussed.

The Durham adaptive optics real-time controller: Capability and ELT suitability

astro-ph — Tue, 22 May 2012 00:44:41 +0000

The Durham adaptive optics real-time controller is a generic, high performance real-time control system for astronomical adaptive optics systems. It has recently had new features added as well as performance improvements, and here we give details of these, as well as ways in which optimisations can be made for specific adaptive optics systems and hardware implementations. We also present new measurements that show how this real-time control system could be used with any existing adaptive optics system, and also show that when used with modern hardware, it has high enough performance to be used with most Extremely Large Telescope adaptive optics systems.

VLSS Redux: Software Improvements applied to the Very Large Array Low-frequency Sky Survey

astro-ph — Tue, 22 May 2012 00:39:29 +0000

We present details of improvements to data processing and analysis which were recently used for a re-reduction of the Very Large Array (VLA) Low-frequency Sky Survey (VLSS) data. Algorithms described are implemented in the data-reduction package Obit, and include smart-windowing to reduce clean bias, improved automatic radio frequency interference removal, improved bright-source peeling, and higher-order Zernike fits to model the ionospheric phase contributions. An additional, but less technical improvement was using the original VLSS catalog as a same-frequency/same-resolution reference for calculating ionospheric corrections, allowing more accuracy and a higher percentage of data for which solutions are found. We also discuss new algorithms for extracting a source catalog and analyzing ionospheric fluctuations present in the data. The improved reduction techniques led to substantial improvements including images of six previously unpublished fields (1% of the survey area) and reducing the clean bias by 50%. The largest angular size imaged has been roughly doubled, and the number of cataloged sources is increased by 35% to 95,000.

Innovative Demodulation Scheme for Coherent Detectors in CMB Experiments [Replacement]

astro-ph — Tue, 22 May 2012 00:33:12 +0000

We propose an innovative demodulation scheme for coherent detectors used in cosmic microwave background polarization experiments. Removal of non-white noise, e.g., narrow-band noise, in detectors is one of the key requirements for the experiments. A combination of modulation and demodulation is used to extract polarization signals as well as to suppress such noise. Traditional demodulation, which is based on the two- point numerical differentiation, works as a first-order high pass filter for the noise. The proposed demodulation is based on the three-point numerical differentiation. It works as a second-order high pass filter. By using a real detector, we confirmed significant improvements of suppression power for the narrow-band noise. We also found improvement of the noise floor.

An Interface for the Virtual Observatory of the University of Guanajuato

astro-ph — Mon, 21 May 2012 00:40:57 +0000

We present the first attempts to build a user-friendly interface for the Virtual Observatory of the University of Guanajuato. The data tables will be accessible to the public through PHP scripts and SQL database managers, such as MySQL and PostgreSQL, all administrated through phpMyAdmin and pgMyAdmin. Although it is not made public yet, this interface will be the basis upon which the final front end for our VO will be built. Furthermore, we present a preliminary version of a web front end to the publicly available stellar population synthesis code STARLIGHT (starlight.ufsc.br) which will be made available with our VO. This front end aims to provide an easy and flexible access to the code itself, letting users fit their own observed spectra with their preferred combination of physical and technical parameters, rather than making available only the results of fitting a specific sample of spectra with predefined parameters.

A divergence-cleaning scheme for cosmological SPMHD simulations

astro-ph — Mon, 21 May 2012 00:37:35 +0000

In magnetohydrodynamics (MHD), the magnetic field is evolved by the induction equation and coupled to the gas dynamics by the Lorentz force. We perform numerical smoothed particle magnetohydrodynamics (Spmhd) simulations and study the influence of a numerical magnetic divergence. For instabilities arising from divergence B related errors, we find the hyperbolic/parabolic cleaning scheme suggested by Dedner et al. 2002 to give good results and prevent numerical artifacts from growing. Additionally, we demonstrate that certain current Spmhd implementations of magnetic field regularizations give rise to unphysical instabilities in long-time simulations. We also find this effect when employing Euler potentials (divergenceless by definition), which are not able to follow the winding-up process of magnetic field lines properly. Furthermore, we present cosmological simulations of galaxy cluster formation at extremely high resolution including the evolution of magnetic fields. We show synthetic Faraday rotation maps and derive structure functions to compare them with observations. Comparing all the simulations with and without divergence cleaning, we are able to confirm the results of previous simulations performed with the standard implementation of MHD in Spmhd at normal resolution. However, at extremely high resolution, a cleaning scheme is needed to prevent the growth of numerical errors at small scales.

Optimization by Smoothed Bandpass Calibration in Radio Spectroscopy [Replacement]

astro-ph — Mon, 21 May 2012 00:31:20 +0000

We have developed the Smoothed Bandpass Calibration (SBC) method and the best suitable scan pattern to optimize radio spectroscopic observations. Adequate spectral smoothing is applied to the spectrum toward OFF-source blank sky adjacent to a target source direction for the purpose of bandpass correction. Because the smoothing process reduces noise, the integration time for OFF-source scans can be reduced keeping the signal-to-noise ratio. Since the smoothing is not applied to ON-source scans, the spectral resolution for line features is kept. An optimal smoothing window is determined by bandpass flatness evaluated by Spectral Allan Variance (SAV). An efficient scan pattern is designed to the OFF-source scans within the bandpass stability timescale estimated by Time-based Allan Variance (TAV). We have tested the SBC using the digital spectrometer, VESPA, on the VERA Iriki station. For the targeted noise level of 5e-4 as a ratio to the system noise, the optimal smoothing window was 32 – 60 ch in the whole bandwidth of 1024 ch, and the optimal scan pattern was designed as a sequence of 70-s ON + 10-s OFF scan pairs. The noise level with the SBC was reduced by a factor of 1.74 compared with the conventional method. The total telescope time to achieve the goal with the SBC was 400 s, which was 1/3 of 1200 s required by the conventional way. Improvement in telescope time efficiency with the SBC was calculated as 3x, 2x and 1.3x for single-beam, dual-beam, and on-the-fly (OTF) scans, respectively. The SBC works to optimize scan patterns for observations from now, and also works to improve signal-to-noise ratios of archival data if ON- and OFF-source spectra are individually recorded, though the efficiency depends on the spectral stability of the receiving system.

Improvement of xenon purification system using a combination of a pulse tube refrigerator and a coaxial heat exchanger

astro-ph — Fri, 18 May 2012 00:47:35 +0000

We have developed a compact cryogenic system with a pulse tube refrigerator and a coaxial heat exchanger. This liquefaction-purification system not only saves the cooling power used to reach high gaseous recirculation rate, but also reduces the impurity level with high speed. The heat exchanger operates with an efficiency of 99%, which indicates the possibility for fast xenon gas recirculation in a highpressurized large-scale xenon storage with much less thermal losses.

Optimization of Bandpass Calibration in Radio Spectroscopy

astro-ph — Fri, 18 May 2012 00:46:50 +0000

INTEGRAL/IBIS 9-year Galactic Hard X-Ray Survey

astro-ph — Fri, 18 May 2012 00:45:18 +0000

Context. The INTEGRAL observatory operating in a hard X-ray/gamma domain gathered a large observational data set over nine years since 2003. Dominant fraction of the observing time was dedicated to the Galactic source population study making the possibility of the deepest Galactic survey in hard X-rays ever compiled. Aims. The aim of the current Galactic survey is to make a basis for Galactic source population studies, and perform mapping of the Milky Way in hard X-rays over the maximum exposure available at |b|<17.5 deg. Methods. We used sky reconstruction algorithms specially developed for the high quality imaging of INTEGRAL/IBIS data. Results. We present sky images, sensitivity maps, and catalogs of detected sources in three energy bands: 17-60, 17-35, and 35-80 keV in the Galactic plane at |b|5 sigma has an identification completeness of ~91%, which is valuable for population studies.

Optimization of the Collection Efficiency of a Hexagonal Light Collector using Quadratic and Cubic B\’ezier Curves

astro-ph — Fri, 18 May 2012 00:43:27 +0000

Reflective light collectors with hexagonal entrance and exit apertures are frequently used in front of the focal-plane camera of a very-high-energy gamma-ray telescope to increase the collection efficiency of atmospheric Cherenkov photons and reduce the night-sky background entering at large incident angles. The shape of a hexagonal light collector is usually based on Winston’s design, which is optimized for only two-dimensional optical systems. However, it is not known whether a hexagonal Winston cone is optimal for the real three-dimensional optical systems of gamma-ray telescopes. For the first time we optimize the shape of a hexagonal light collector using quadratic and cubic B\’ezier curves. We demonstrate that our optimized designs simultaneously achieve a higher collection efficiency and background reduction rate than traditional designs.

A New Method for Cross Polarized Delay Calibration of Radio Interferometers

astro-ph — Fri, 18 May 2012 00:41:40 +0000

Radio interferometers can measure the full polarization state of incoming waves by cross–correlating all combinations of two orthogonal polarizations at each antenna. The independent sets of electronics used to detect the two polarization states will introduce a differential instrumental delay between the two data streams. The usual technique of separate calibration of the parallel–hand sets of visibilities still allows for an arbitrary offset in group delay and phase between the two parallel systems. In order to use the cross–polarized visibilities, this instrumental offset must be determined and removed. This paper describes one such technique and explores its application in the Obit package. The technique is successfully applied to some EVLA data using both strongly and weakly polarized calibrators.

Approximate Bayesian Computation for Astronomical Model Analysis: A Case Study in Galaxy Demographics and Morphological Transformation at High Redshift [Replacement]

astro-ph — Fri, 18 May 2012 00:34:08 +0000

“Approximate Bayesian Computation” (ABC) represents a powerful methodology for the analysis of complex stochastic systems for which the likelihood of the observed data under an arbitrary set of input parameters may be entirely intractable-the latter condition rendering useless the standard machinery of tractable likelihood-based, Bayesian statistical inference (e.g. conventional Markov Chain Monte Carlo simulation; MCMC). In this article we demonstrate the potential of ABC for astronomical model analysis by application to a case study in the morphological transformation of high redshift galaxies. To this end we develop, first, a stochastic model for the competing processes of merging and secular evolution in the early Universe; and second, through an ABC-based comparison against the observed demographics of massive (M_gal > 10^11 M_sun) galaxies (at 1.5 < z < 3) in the CANDELS/EGS dataset we derive posterior probability densities for the key parameters of this model. The "Sequential Monte Carlo" (SMC) implementation of ABC exhibited herein, featuring both a self-generating target sequence and self-refining MCMC kernel, is amongst the most efficient of contemporary approaches to this important statistical algorithm. We highlight as well through our chosen case study the value of careful summary statistic selection, and demonstrate two modern strategies for assessment and optimisation in this regard. Ultimately, our ABC analysis of the high redshift morphological mix returns tight constraints on the evolving merger rate in the early Universe and favours major merging (with disc survival or rapid reformation) over secular evolution as the mechanism most responsible for building up the first generation of bulges in early-type disks.

Paraiso : An Automated Tuning Framework for Explicit Solvers of Partial Differential Equations [Replacement]

astro-ph — Fri, 18 May 2012 00:32:27 +0000

We propose Paraiso, a domain specific language embedded in functional programming language Haskell, for automated tuning of explicit solvers of partial differential equations (PDEs) on GPUs as well as multicore CPUs. In Paraiso, one can describe PDE solving algorithms succinctly using tensor equations notation. Hydrodynamic properties, interpolation methods and other building blocks are described in abstract, modular, re-usable and combinable forms, which lets us generate versatile solvers from little set of Paraiso source codes. We demonstrate Paraiso by implementing a compressive hydrodynamics solver. A single source code less than 500 lines can be used to generate solvers of arbitrary dimensions, for both multicore CPUs and GPUs. We demonstrate both manual annotation based tuning and evolutionary computing based automated tuning of the program.

EzGal: A Flexible Interface for Stellar Population Synthesis Models [Replacement]

astro-ph — Fri, 18 May 2012 00:32:13 +0000

We present EzGal, a flexible python program designed to easily generate observable parameters (magnitudes, colors, mass-to-light ratios) for any stellar population synthesis (SPS) model. As has been demonstrated by various authors, the choice of input SPS models can be a significant source of systematic uncertainty. A key strength of EzGal is that it enables simple, direct comparison of different models sets. EzGal is also capable of generating composite stellar population models (CSPs) and can interpolate between metallicities for a given model set. We have created a web interface to run EzGal and generate observables for a variety of star formation histories and model sets. We make many commonly used SPS models available from this interface; the BC03 models, an updated version of these models, the Maraston models, the BaSTI models, and finally the FSPS models. We use EzGal to compare magnitude predictions for the model sets as a function of wavelength, age, metallicity, and star formation history. We recover the well-known result that the models agree best in the optical for old, solar metallicity models, with differences at the ~0.1 magnitude level. The most problematic regime for SPS modeling is for young ages (7500 Angstroms) where scatter between models can vary from 0.3 mags (Sloan i) to 0.7 mags (Ks). We find that these differences are best understood as general uncertainties in SPS modeling. Finally we explore a more physically motivated example by generating CSPs with a star formation history matching the global star formation history of the universe. We demonstrate that the wavelength and age dependence of SPS model uncertainty translates into a redshift dependent model uncertainty, highlighting the importance of a quantitative understanding of model differences when comparing observations to models as a function of redshift.

The Black Hole Evolution and Space Time (BEST) Observatory

astro-ph — Thu, 17 May 2012 00:45:47 +0000

In this white paper, we discuss the concept of a next-generation X-ray mission called BEST (Black hole Evolution and Space Time). The mission concept uses a 3000 square centimeter effective area mirror (at 6 keV) to achieve unprecedented sensitivities for hard X-ray imaging spectrometry (5-70 keV) and for broadband X-ray polarimetry (2-70 keV). BEST can make substantial contributions to our understanding of the inner workings of accreting black holes, our knowledge about the fabric of extremely curved spacetime, and the evolution of supermassive black holes. BEST will allow for time resolved studies of accretion disks. With a more than seven times larger mirror area and a seven times wider bandpass than GEMS, BEST will take X-ray polarimetry to a new level: it will probe the time variability of the X-ray polarization from stellar mass and supermassive black holes, and it will measure the polarization properties in 30 independent energy bins. These capabilities will allow BEST to conduct tests of accretion disk models and the underlying spacetimes. With three times larger mirror area and ten times better angular resolution than NuSTAR, BEST will be able to make deep field observations with a more than 15 times better sensitivity than NuSTAR. The mission will be able to trace the evolution of obscured and unobscured black holes in the redshift range from zero to six, covering the most important epoch of supermassive black hole growth. The hard X-ray sensitivity of BEST will enable a deep census of non-thermal particle populations. BEST will give us insights into AGN feedback by measuring the particle luminosity injected by AGNs into the interstellar medium (ISM) of their hosts, and will map the emission from particles accelerated at large scale structure shocks. Finally, BEST has the potential to constrain the equation of state of neutron stars (NS).

Starship Sails Propelled by Cost-Optimized Directed Energy [Replacement]

astro-ph — Thu, 17 May 2012 00:38:43 +0000

Microwave propelled sails are a new class of spacecraft using photon acceleration. It is the only method of interstellar flight that has no physics issues. Laboratory demonstrations of basic features of beam-driven propulsion, flight, stability (‘beam-riding’), and induced spin, have been completed in the last decade, primarily in the microwave. It offers much lower cost probes after a substantial investment in the launcher. Engineering issues are being addressed by other applications: fusion (microwave, millimeter and laser sources) and astronomy (large aperture antennas). There are many candidate sail materials: carbon nanotubes and microtrusses, graphene, beryllium, etc. For acceleration of a sail, what is the cost-optimum high power system? Here the cost is used to constrain design parameters to estimate system power, aperture and elements of capital and operating cost. From general relations for cost-optimal transmitter aperture and power, system cost scales with kinetic energy and inversely with sail diameter and frequency. So optimal sails will be larger, lower in mass and driven by higher frequency beams. Estimated costs include economies of scale. We present several starship point concepts. Systems based on microwave, millimeter wave and laser technologies are of equal cost at today’s costs. The frequency advantage of lasers is cancelled by the high cost of both the laser and the radiating optic.

The LWA1 Radio Telescope [Replacement]

astro-ph — Thu, 17 May 2012 00:34:12 +0000

LWA1 is a new radio telescope operating in the frequency range 10-88 MHz, located in central New Mexico. The telescope consists of 258 pairs of dipole-type antennas whose outputs are individually digitized and formed into beams. Simultaneously, signals from all dipoles can be recorded using one of the instrument’s “all dipoles” modes, facilitating all-sky imaging. Notable features of the instrument include high intrinsic sensitivity (about 6 kJy zenith system equivalent flux density), large instantaneous bandwidth (up to 78 MHz), and 4 independently-steerable beams utilizing digital “true time delay” beamforming. This paper summarizes the design of LWA1 and its performance as determined in commissioning experiments. We describe the method currently in use for array calibration, and report on measurements of sensitivity and beamwidth.

Non-local thermodynamic equilibrium inversions from a 3D MHD chromospheric model

astro-ph — Wed, 16 May 2012 00:59:36 +0000

The structure of the solar chromosphere is believed to be governed by magnetic fields, even in quiet-Sun regions that have a relatively weak photospheric field. During the past decade inversion methods have emerged as powerful tools for analyzing the chromosphere of active regions. The applicability of inversions to infer the stratification of the physical conditions in a dynamic 3D solar chromosphere has not yet been studied in detail. This study aims to establish the diagnostic capabilities of non-local thermodynamical equilibrium (NLTE) inversion techniques of Stokes profiles induced by the Zeeman effect in the Ca II 8542 line. We computed the Ca II atomic level populations in a snapshot from a 3D radiation-MHD simulation of the quiet solar atmosphere in non-LTE using the 3D radiative transfer code Multi3d. These populations were used to compute synthetic full-Stokes profiles in the Ca II 8542 line using 1.5D radiative transfer and the inversion code Nicole. The profiles were then spectrally degraded to account for finite filter width and Gaussian noise was added to account for finite photon flux. These profiles were inverted using Nicole and the results were compared with the original model atmosphere. Our NLTE inversions applied to quiet-Sun synthetic observations provide reasonably good estimates of the chromospheric magnetic field, line-of-sight velocities and somewhat less accurate, but still very useful, estimates of the temperature. Three dimensional scattering of photons cause cool pockets in the chromosphere to be invisible in the line profile and consequently they are also not recovered by the inversions. To successfully detect Stokes linear polarization in this quiet snapshot, a noise level below 10^{-3.5} is necessary.

Balanced Homodyne Detection of Optical Quantum States at Audio-Band Frequencies and Below [Cross-Listing]

astro-ph — Wed, 16 May 2012 00:49:36 +0000

The advent of stable, highly squeezed states of light has generated great interest in the gravitational wave community as a means for improving the quantumnoise- limited performance of advanced interferometric detectors. To confidently measure these squeezed states, it is first necessary to measure the shot-noise across the frequency band of interest. Technical noise, such as non-stationary events, beam pointing, and parasitic interference, can corrupt shot-noise measurements at low Fourier frequencies, below tens of kilo-Hertz. In this paper we present a qualitative investigation into all of the relevant noise sources and the methods by which they can be identified and mitigated in order to achieve quantum noise limited balanced homodyne detection. Using these techniques, flat shot-noise down to Fourier frequencies below 0.5 Hz is produced. This enables the direct observation of large magnitudes of squeezing across the entire audio-band, of particular interest for ground-based interferometric gravitational wave detectors. 11.6 dB of shot-noise suppression is directly observed, with more than 10 dB down to 10 Hz.

AGN Physics with the Cherenkov Telescope Array [Replacement]

astro-ph — Wed, 16 May 2012 00:32:35 +0000

The Cherenkov Telescope Array (CTA), currently in its Preparatory Phase, will be the first open observatory for very high energy gamma-rays from galactic and extragalactic sources. The international consortium behind CTA is preparing the construction of two large arrays of Cherenkov telescopes in the Northern and Southern Hemispheres with a performance that will be significantly improved compared to the current generation of arrays. Its increased sensitivity and energy range will give CTA access to a large population of Active Galactic Nuclei (AGN) not yet detected at very high energies and provide much more details on known TeV sources. While the low end of the CTA energy coverage will close the current gap with the Fermi-LAT band, its high energy coverage will open a new window on the sky and help us understand the intrinsic shape of the hardest blazar spectra. We outline the current status of CTA and discuss the science case for AGN physics with the observatory. Predictions for source detections based on extrapolations of Fermi-LAT spectra are discussed. An overview is given of prospects for the detection of extended emission from radio galaxies, of rapid variability, and spectral features. The observation of AGN with CTA will also improve current constraints on the distribution of the extragalactic background light, the strength of the intergalactic magnetic field and Lorentz invariance violation.

An Efficient Parameter Space Search as an Alternative to Markov Chain Monte Carlo

astro-ph — Tue, 15 May 2012 00:51:53 +0000

We consider the problem of inferring constraints on a high-dimensional parameter space with a computationally expensive likelihood function. Markov chain Monte Carlo (MCMC) methods offer significant improvements in efficiency over grid-based searches and are easy to implement in a wide range of cases. However, MCMCs offer few guarantees that all of the interesting regions of parameter space are explored. We propose a machine learning algorithm that improves upon the performance of MCMC by intelligently targeting likelihood evaluations so as to quickly and accurately characterize the likelihood surface in both low- and high-likelihood regions. We compare our algorithm to MCMC on toy examples and the 7-year WMAP cosmic microwave background data release. Our algorithm finds comparable parameter constraints to MCMC in fewer calls to the likelihood function and with greater certainty that all of the interesting regions of parameter space have been explored.

Gravitational softening as a smoothing operation

astro-ph — Tue, 15 May 2012 00:50:49 +0000

In self-consistent N-body simulations of collisionless systems, gravitational interactions are modified on small scales to remove singularities and simplify the task of numerically integrating the equations of motion. This `gravitational softening’ is sometimes presented as an ad-hoc departure from Newtonian gravity. However, softening can also be described as a smoothing operation applied to the mass distribution; the gravitational potential and the smoothed density obey Poisson’s equation precisely. While `softening’ and `smoothing’ are mathematically equivalent descriptions, the latter has some advantages. For example, the smoothing description suggests a way to set up N-body initial conditions in almost perfect dynamical equilibrium.

Space-quality data from balloon-borne telescopes: the High Altitude Lensing Observatory (HALO)

astro-ph — Tue, 15 May 2012 00:43:53 +0000

We present a method for attaining sub-arcsecond pointing stability during sub- orbital balloon flights, as designed for in the High Altitude Lensing Observatory (HALO) concept. The pointing method presented here has the potential to perform near-space quality optical astronomical imaging at 1-2% of the cost of space-based missions. We also discuss an architecture that can achieve sufficient thermomechanical stability to match the pointing stability. This concept is motivated by advances in the development and testing of Ultra Long Duration Balloon (ULDB) flights which promise to allow observation campaigns lasting more than three months. The design incorporates a multi-stage pointing architecture comprising: a gondola coarse azimuth control system, a multi-axis nested gimbal frame structure with arcsecond stability, a telescope de-rotator to eliminate field rotation, and a fine guidance stage consisting of both a telescope mounted angular rate sensor and guide CCDs in the focal plane to drive a fast-steering mirror. We discuss the results of pointing tests together with a preliminary thermo-mechanical analysis required for sub-arcsecond pointing at high altitude. Possible future applications in the areas of wide-field surveys and exoplanet searches are also discussed.

Estimate of the impact of background particles on the X-Ray Microcalorimeter Spectrometer on IXO

astro-ph — Tue, 15 May 2012 00:42:38 +0000

We present the results of a study on the impact of particles of galactic (GCR) and solar origin for the X-ray Microcalorimeter Spectrometer (XMS) aboard an astronomical satellite flying in an orbit at the second Lagrangian point (L2). The detailed configuration presented in this paper is the one adopted for the International X-Ray Observatory (IXO) study, however the derived estimates can be considered a conservative limit for ATHENA, that is the IXO redefined mission proposed to ESA. This work is aimed at the estimate of the residual background level expected on the focal plane detector during the mission lifetime, a crucial information in the development of any instrumental configuration that optimizes the XMS scientific performances. We used the Geant4 toolkit, a Monte Carlo based simulator, to investigate the rejection efficiency of the anticoincidence system and assess the residual background on the detector.

AARTFAAC: Towards a 24×7, All-sky Monitor for LOFAR

astro-ph — Tue, 15 May 2012 00:40:17 +0000

The AARTFAAC project aims to implement an All-Sky Monitor (ASM), using the Low Frequency Array (LOFAR) telescope. It will enable real-time, 24×7 monitoring for low frequency radio transients over most of the sky locally visible to the LOFAR at timescales ranging from milliseconds to several days, and rapid triggering of follow-up observations with the full LOFAR on detection of potential transient candidates. These requirements pose several implementation challenges: imaging of an all-sky field of view, low latencies of processing, continuous availability and autonomous operation of the ASM. The first of these has already resulted in the correlator for the ASM being the largest in the world in terms of its number of input channels. It will generate $\sim 1.5 \cdot 10^5$ correlations per second per spectral channel when built. Test observations using existing LOFAR infrastructure were carried out to quantify and constrain crucial instrumental design criteria for the ASM. In this paper, we present an overview of the AARTFAAC data processing pipeline and illustrate some of the aforementioned challenges by showing all-sky images obtained from one of the test observations. These results provide quantitative estimates of the capabilities of the instrument.

Treatment of Calibration Uncertainty in Multi-Baseline Cross-Correlation Searches for Gravitational Waves [Cross-Listing]

astro-ph — Tue, 15 May 2012 00:38:25 +0000

Residual uncertainty in the calibration of gravitational wave (GW) detector data leads to systematic errors which must be accounted for in setting limits on the strength of GW signals. When cross-correlation measurements are made using data from a pair of instruments, as in searches for a stochastic GW background, the calibration uncertainties associated with the two instruments can be combined into an uncertainty associated with the pair. With the advent of multi-baseline GW observation (e.g., networks consisting of multiple detectors such as the LIGO observatories and Virgo), a more sophisticated treatment is called for. We describe how the correlations between calibration factors associated with different pairs can be taken into account by marginalizing over the uncertainty associated with each instrument, defining two methods known as per-baseline and per-instrument marginalization.

Signal transceiver transit times and propagation delay corrections for ranging and geo-referencing applications

astro-ph — Tue, 15 May 2012 00:38:14 +0000

The changes in phase, time and frequency suffered by signals when retransmitted by a remote and inaccessible transponder and the propagation delays are major constraints to obtain accurate ranging measurements in various related applications. We present a new method and system to determine these delays for every single pulsed signal transmission. The process utilizes four ground-based reference stations, synchronized in time and installed at well known geodesic coordinates. The repeater station is located within the fields of view common to the four reference bases, such as in a platform transported by a satellite, balloon, aircraft, etc. Signal transmitted by one of the reference bases is retransmitted by the transponder, received back by the four bases, producing four ranging measurements which are processed to determine uniquely the time delays undergone in every retransmission process. The repeater’s positions with respect to each group of three out of four reference bases are given by a system of equations. A minimization function is derived comparing repeater’s positions referred to at least two groups of three reference bases. The minimum found by iterative methods provide the signal transit time at the repeater and propagation delays, providing the correct repeater position. The method is applicable to the transponder platform positioning and navigation, time synchronization of remote clocks, and location of targets. The algorithm has been fully demonstrated simulated for practical situation with the transponder carried by an aircraft moving over bases on the ground. The errors of the determinations have been evaluated for uncertainties in clock synchronization, in propagation time delays and other system parameters.

Sparsity Averaging Reweighted Analysis (SARA): a novel algorithm for radio-interferometric imaging

astro-ph — Tue, 15 May 2012 00:38:05 +0000

We propose a novel algorithm for image reconstruction in radio interferometry. The ill-posed inverse problem associated with the incomplete Fourier sampling identified by the visibility measurements, is regularized by the assumption of average signal sparsity over representations in multiple wavelet bases. The algorithm, defined in the versatile framework of convex optimization, is dubbed Sparsity Averaging Reweighted Analysis (SARA). We show through simulations that the proposed approach largely outperforms state-of-the-art imaging methods in the field, which are based on the assumption of signal sparsity in a single basis only.

Evaluation of the discovery potential of an underwater Mediterranean neutrino telescope taking into account the estimated directional resolution and energy of the reconstructed tracks [Replacement]

astro-ph — Tue, 15 May 2012 00:35:07 +0000

We report on the development of search methods for point-like and extended neutrino sources, utilizing the tracking and energy estimation capabilities of an underwater, Very Large Volume Neutrino Telescope (VLVnT). We demonstrate that the developed techniques offer a significant improvement on the telescope’s discovery potential. We also present results on the potential of the Mediterranean KM3NeT to discover galactic neutrino sources.

The GJ 436 System: Directly Determined Astrophysical Parameters of an M-Dwarf and Implications for the Transiting Hot Neptune [Replacement]

astro-ph — Tue, 15 May 2012 00:34:50 +0000

The late-type dwarf GJ 436 is known to host a transiting Neptune-mass planet in a 2.6-day orbit. We present results of our interferometric measurements to directly determine the stellar diameter ($R_{\star} = 0.455 \pm 0.018 R_{\odot}$) and effective temperature ($T_{\rm EFF} = 3416 \pm 54$ K). We combine our stellar parameters with literature time-series data, which allows us to calculate physical and orbital system parameters, including GJ 436’s stellar mass ($M_{\star} = 0.507^{+ 0.071}_{- 0.062} M_{\odot}$) and density ($\rho_* = 5.37^{+ 0.30}_{- 0.27} \rho_\odot$), planetary radius ($R_{p} = 0.369^{+ 0.015}_{- 0.015} R_{Jupiter}$), planetary mass ($M_{p} = 0.078^{+ 0.007}_{- 0.008} M_{Jupiter}$), implying a mean planetary density of $\rho_{p} = 1.55^{+ 0.12}_{- 0.10} \rho_{Jupiter}$. These values are generally in good agreement with previous literature estimates based on assumed stellar mass and photometric light curve fitting. Finally, we examine the expected phase curves of the hot Neptune GJ 436b, based on various assumptions concerning the efficiency of energy redistribution in the planetary atmosphere, and find that it could be constrained with {\it Spitzer} monitoring observations.

Detection of Solar Rotational Variability in the LYRA 190 – 222 nm Spectral Band

astro-ph — Mon, 14 May 2012 00:52:39 +0000

We analyze the variability of the spectral solar irradiance during the period from 7 January, 2010 until 20 January, 2010 as measured by the Herzberg channel (190-222 nm) of the Large Yield RAdiometer (LYRA) onboard PROBA2. In this period of time observations by the LYRA nominal unit experienced degradation and the signal produced by the Herzberg channel frequently jumped from one level to another. Both these factors significantly complicates the analysis. We present the algorithm which allowed us to extract the solar variability from the LYRA data and compare the results with SORCE/SOLSTICE measurements and with modeling based on the Code for the Solar Irradiance (COSI).

Iris: The VAO SED Application

astro-ph — Mon, 14 May 2012 00:47:24 +0000

We present Iris, the VAO (Virtual Astronomical Observatory) application for analyzing SEDs (spectral energy distributions). Iris is the result of one of the major science initiatives of the VAO, and the first version was released in September 2011. Iris combines key features of several existing software applications to streamline and enhance SED analysis. With Iris, users may read and display SEDs, select data ranges for analysis, fit models to SEDs, and calculate confidence limits on best-fit parameters. SED data may be uploaded into the application from IVOA-compliant VOTable and FITS format files, or retrieved directly from NED. Data written in unsupported formats may be converted using SedImporter, a new application provided with Iris. The components of Iris have been contributed by members of the VAO. Specview, contributed by STScI, provides a GUI for reading, editing, and displaying SEDs, as well as defining models and parameter values. Sherpa, contributed by the Chandra project at SAO, provides a library of models, fit statistics, and optimization methods; the underlying I/O library, SEDLib, is a VAO product written by SAO to current IVOA (International Virtual Observatory Alliance) data model standards. NED is a service provided by IPAC for easy location of data for a given extragalactic source, including SEDs. SedImporter is a new tool for converting non-standard SED data files into a format supported by Iris. We demonstrate the use of SedImporter to retrieve SEDs from a variety of sources–from the NED SED service, from the user’s own data, and from other VO applications using SAMP (Simple Application Messaging Protocol). We also demonstrate the use of Iris to read, display, select ranges from, and fit models to SEDs. Finally, we discuss the architecture of Iris, and the use of IVOA standards so that Specview, Sherpa, SEDLib and SedImporter work together seamlessly.

Building a VO-compliant Radio Astronomical DAta Model for Single-dish radio telescopes (RADAMS)

astro-ph — Mon, 14 May 2012 00:38:41 +0000

The Virtual Observatory (VO) is becoming the de-facto standard for astronomical data publication. However, the number of radio astronomical archives is still low in general, and even lower is the number of radio astronomical data available through the VO. In order to facilitate the building of new radio astronomical archives, easing at the same time their interoperability with VO framework, we have developed a VO-compliant data model which provides interoperable data semantics for radio data. That model, which we call the Radio Astronomical DAta Model for Single-dish (RADAMS) has been built using standards of (and recommendations from) the International Virtual Observatory Alliance (IVOA). This article describes the RADAMS and its components, including archived entities and their relationships to VO metadata. We show that by using IVOA principles and concepts, the effort needed for both the development of the archives and their VO compatibility has been lowered, and the joint development of two radio astronomical archives have been possible. We plan to adapt RADAMS to be able to deal with interferometry data in the future.

Scanamorphos: a map-making software for Herschel and similar scanning bolometer arrays

astro-ph — Mon, 14 May 2012 00:38:28 +0000

Scanamorphos is one of the public softwares available to post-process scan observations performed with the Herschel photometer arrays. This post-processing mainly consists in subtracting the total low-frequency noise (both its thermal and non-thermal components), masking cosmic ray hit residuals, and projecting the data onto a map. Although it was developed for Herschel, it is also applicable with minimal adjustment to scan observations made with other bolometer arrays, provided they entail sufficient redundancy; it was successfully applied to P-Artemis, an instrument operating on the APEX telescope. Contrary to most other algorithms (first developed for microwave background experiments and later adapted to Herschel), Scanamorphos does not assume any particular noise model, and does not apply any Fourier-space filtering to the data, but is an empirical tool using purely the redundancy built in the observations — taking advantage of the fact that each portion of the sky is sampled at multiple times by multiple bolometers. It is an interactive software in the sense that the user is allowed to optionally visualize and control results at each intermediate step, but the processing is fully automated. This paper describes the principles and algorithm of Scanamorphos and presents several examples of application.

Interactive Visualization and Simulation of Astronomical Nebulae [Replacement]

astro-ph — Mon, 14 May 2012 00:32:49 +0000

Interactive visualization and simulation of astrophysical phenomena help astronomers and enable digital planetariums and television documentaries to take their spectators on a journey into deep space to explore the astronomical wonders of our universe in 3D.

Data Acquisition, Triggering, and Filtering at the Auger Engineering Radio Array

astro-ph — Fri, 11 May 2012 00:45:48 +0000

The Auger Engineering Radio Array (AERA) is currently detecting cosmic rays of energies at and above 10^17 eV at the Pierre Auger Observatory, by triggering on the radio emission produced in the associated air showers. The radio-detection technique must cope with a significant background of man-made radio-frequency interference, but can provide information on shower development with a high duty cycle. We discuss our techniques to handle the challenges of self-triggered radio detection in a low-power autonomous array, including triggering and filtering algorithms, data acquisition design, and communication systems.

Stellar Polarimetry: Where Are We and Where Are We Going?

astro-ph — Fri, 11 May 2012 00:42:43 +0000

On the final day of the Stellar Polarimetry conference, participants split up into three “breakout sessions” to discuss the future of the field in the areas of instrumentation, upcoming opportunities, and community priorities. This contribution compiles the major recommendations arising from each breakout session. We hope that the polarimetric community will find these ideas useful as we consider how to maintain the vitality of polarimetry in the coming years.

An Ultra-Low Background PMT for Liquid Xenon Detectors [Cross-Listing]

astro-ph — Fri, 11 May 2012 00:38:42 +0000

Results are presented from radioactivity screening of two models of photomultiplier tubes designed for use in current and future liquid xenon experiments. The Hamamatsu 5.6 cm diameter R8778 PMT, used in the LUX dark matter experiment, has yielded a positive detection of four common radioactive isotopes: 238U, 232Th, 40K, and 60Co. Screening of LUX materials has rendered backgrounds from other detector materials subdominant to the R8778 contribution. A prototype Hamamatsu 7.6 cm diameter R11410 MOD PMT has also been screened, with benchmark isotope counts measured at <0.4 238 U / <0.3 232 Th / <8.3 40 K / 2.0+-0.2 60 Co mBq/PMT. This represents a large reduction, equal to a change of \times 1/24 238U / \times 1/9 232Th / \times 1/8 40K per PMT, between R8778 and R11410 MOD, concurrent with a doubling of the photocathode surface area (4.5 cm to 6.4 cm diameter). 60Co measurements are comparable between the PMTs, but can be significantly reduced in future R11410 MOD units through further material selection. Assuming PMT activity equal to the measured 90% upper limits, Monte Carlo estimates indicate that replacement of R8778 PMTs with R11410 MOD PMTs will change LUX PMT electron recoil background contributions by a factor of \times1/25 after further material selection for 60Co reduction, and nuclear recoil backgrounds by a factor of \times 1/36. The strong reduction in backgrounds below the measured R8778 levels makes the R11410 MOD a very competitive technology for use in large-scale liquid xenon detectors.

A Dynamic Era-Based Time-Symmetric Block Time-Step Algorithm with Parallel Implementations

astro-ph — Fri, 11 May 2012 00:37:58 +0000

The time-symmetric block time–step (TSBTS) algorithm is a newly developed efficient scheme for $N$–body integrations. It is constructed on an era-based iteration. In this work, we re-designed the TSBTS integration scheme with dynamically changing era size. A number of numerical tests were performed to show the importance of choosing the size of the era, especially for long time integrations. Our second aim was to show that the TSBTS scheme is as suitable as previously known schemes for developing parallel $N$–body codes. In this work, we relied on a parallel scheme using the copy algorithm for the time-symmetric scheme. We implemented a hybrid of data and task parallelization for force calculation to handle load balancing problems that can appear in practice. Using the Plummer model initial conditions for different numbers of particles, we obtained the expected efficiency and speedup for a small number of particles. Although parallelization of the direct $N$–body codes is negatively affected by the communication/calculation ratios, we obtained good load balance results. Moreover, we were able to conserve the advantages of the algorithm (e.g., energy conservation for long–term simulations).

A Well-Posed Kelvin-Helmholtz Instability Test and Comparison [Replacement]

astro-ph — Fri, 11 May 2012 00:34:44 +0000

Recently, there has been a significant level of discussion of the correct treatment of Kelvin-Helmholtz instability in the astrophysical community. This discussion relies largely on how the KHI test is posed and analyzed. We pose a stringent test of the initial growth of the instability. The goal is to provide a rigorous methodology for verifying a code on two dimensional Kelvin-Helmholtz instability. We ran the problem in the Pencil Code, Athena, Enzo, NDSPHMHD, and Phurbas. A strict comparison, judgment, or ranking, between codes is beyond the scope of this work, though this work provides the mathematical framework needed for such a study. Nonetheless, how the test is posed circumvents the issues raised by tests starting from a sharp contact discontinuity yet it still shows the poor performance of Smoothed Particle Hydrodynamics. We then comment on the connection between this behavior to the underlying lack of zeroth-order consistency in Smoothed Particle Hydrodynamics interpolation. We comment on the tendency of some methods, particularly those with very low numerical diffusion, to produce secondary Kelvin-Helmholtz billows on similar tests. Though the lack of a fixed, physical diffusive scale in the Euler equations lies at the root of the issue, we suggest that in some methods an extra diffusion operator should be used to damp the growth of instabilities arising from grid noise. This statement applies particularly to moving-mesh tessellation codes, but also to fixed-grid Godunov schemes.

Improved visual detection of moving objects in astronomical images using color intensity projections with hue cycling [Replacement]

astro-ph — Fri, 11 May 2012 00:34:00 +0000

While fully automated methods for detecting faint moving objects in astronomical images – such as Kuiper belt objects (KBOs) – are constantly improving, visual detection still has a role to play especially when the fixed background is cluttered with stars. Color intensity projections (CIPs) using hue cycling – which combines a sequence of greyscale images into a single color image – aids in the visual detection of moving objects by highlighting them using color in an intuitive way. To demonstrate the usefulness of CIPs in detecting faint moving objects a sequence of 16 images from the SuprimeCam camera of the Subaru telescope were combined into a CIPs image. As well has making even faint moving objects easier to visually detect against a cluttered background, CCD artefacts were also more easily recognisable. The new Hyper SuprimeCam for the Subaru telescope – which will allow many short exposure images to be acquired with little dead time between images – should provide ideal data for use with the CIPs algorithm. In addition, the current search for KBOs to be targeted by the New Horizon’s spacecraft after its flyby of Pluto provides an excellent test case for the state of the art in faint moving object detection against a cluttered background.

Testing a hypothesis of the \nu Octantis planetary system [Replacement]

astro-ph — Fri, 11 May 2012 00:31:42 +0000

We investigate the orbital stability of a putative Jovian planet reported by Ramm et. al in a compact binary \nu Octantis. Our numerical study makes use of a new computational Message Passing Interface (MPI) framework Mechanic which we developed to run massive numerical experiments on CPU clusters. The code is illustrated on a model Hamiltonian introduced by Froeschl\’e et al. We confirm that the \nu Octantis planet could reside in a retrograde orbit, according with a hypothesis of Eberle & Cuntz. It may be present in a zone of stable motions which has a structure of the Arnold web formed due to overlapping of low-order mean motion resonances and their sub-resonances. We also re-analyzed the available radial velocity data in terms of self-consistent Newtonian N-body model. We found stable best-fit solutions that obey the observational constraints. They correspond to retrograde, strictly anti-aligned orbits of the binary and the planet. However, these solutions are confined in very small stable regions of the phase space. The presence of a real planet in the system is still questionable, because its formation would be hindered by strong dynamical perturbations.

Improved Variable Star Search in Large Photometric Data Sets — New Variables in CoRoT Field LRa02 Detected by BEST II [Replacement]

astro-ph — Fri, 11 May 2012 00:31:15 +0000

The CoRoT field LRa02 has been observed with the Berlin Exoplanet Search Telescope II (BEST II) during the southern summer 2007/2008. A first analysis of stellar variability led to the publication of 345 newly discovered variable stars. Now, a deeper analysis of this data set was used to optimize the variability search procedure. Several methods and parameters have been tested in order to improve the selection process compared to the widely used J index for variability ranking. This paper describes an empirical approach to treat systematic trends in photometric data based upon the analysis of variance statistics that can significantly decrease the rate of false detections. Finally, the process of reanalysis and method improvement has virtually doubled the number of variable stars compared to the first analysis by Kabath et al. A supplementary catalog of 272 previously unknown periodic variables plus 52 stars with suspected variability is presented. Improved ephemerides are given for 19 known variables in the field. In addition, the BEST II results are compared with CoRoT data and its automatic variability classification.

Demonstration of Time Delay Interferometry and Spacecraft Ranging in a Space-based Gravitational Wave Detector using the UF-LISA Interferometry Simulator

astro-ph — Thu, 10 May 2012 00:42:35 +0000

Space-based gravitational-wave observatories such as the Laser Interferometer Space Antenna (LISA) use time-shifted and time-scaled linear combinations of differential laser-phase beat signals to cancel the otherwise overwhelming laser frequency noise. Nanosecond timing precision is needed to accurately form these Time-Delay Interferometry (TDI) combinations which defines a ~1 meter requirement on the inter-spacecraft ranging capability. The University of Florida Hardware-in-the-loop LISA Interferometry Simulator (UFLIS) has been used to test Time-Delay Interferometry in a configuration which incorporates variable delays, realistic Doppler shifts, and simulated gravitational-wave signals. The TDI 2.0 combinations are exploited to determine the time-changing delays with nanosecond accuracy using a TDI-ranging reference tone. These variable delays are used in forming the TDI combinations to achieve the LISA interferometry sensitivity resulting from 10 orders of magnitude laser frequency noise cancellation.

Wideband Infrared Spectrometer for Characterization of Transiting Exoplanets with Space Telescopes

astro-ph — Thu, 10 May 2012 00:42:13 +0000

This paper presents a conceptual design for a spectrometer designed specifically for characterizing transiting exoplanets with space-borne infrared telescopes. The design adopting cross-dispersion is intended to be simple, compact, highly stable, and has capability of simultaneous coverage over a wide wavelength region with high throughput. Typical wavelength coverage and spectral resolving power is 1-13 micron with a spectral resolving power of ~ a few hundred, respectively. The baseline design consists of two detectors, two prisms with a dichroic coating and microstructured grating surfaces, and three mirrors. Moving parts are not adopted. The effect of defocusing is evaluated for the case of a simple shift of the detector, and anisotropic defocusing to maintain the spectral resolving power. Variations in the design and its application to planned missions are also discussed.

Improved Variable Star Search in Large Photometric Data Sets — New Variables in CoRoT Field LRa02 Detected ba BEST II

astro-ph — Thu, 10 May 2012 00:41:19 +0000

An atmospheric radiation model for Cerro Paranal. I. The optical spectral range

astro-ph — Thu, 10 May 2012 00:40:17 +0000

The Earth’s atmosphere affects ground-based astronomical observations. Scattering, absorption, and radiation processes deteriorate the signal-to-noise ratio of the data received. For scheduling astronomical observations it is, therefore, important to accurately estimate the wavelength-dependent effect of the Earth’s atmosphere on the observed flux. In order to increase the accuracy of the exposure time calculator of the European Southern Observatory’s (ESO) Very Large Telescope (VLT) at Cerro Paranal, an atmospheric model was developed as part of the Austrian ESO In-Kind contribution. It includes all relevant components, such as scattered moonlight, scattered starlight, zodiacal light, atmospheric thermal radiation and absorption, and non-thermal airglow emission. This paper focuses on atmospheric scattering processes that mostly affect the blue ( 0.55 mum) wavelength regime. While the former is mainly investigated by means of radiative transfer models, the intensity and variability of the latter is studied with a sample of 1186 VLT FORS1 spectra. For a set of parameters such as the object altitude angle, Moon-object angular distance, ecliptic latitude, bimonthly period, and solar radio flux, our model predicts atmospheric radiation and transmission at a requested resolution. A comparison of our model with the FORS1 spectra and photometric data for the night-sky brightness from the literature, suggest a model accuracy of about 20%. This is a significant improvement with respect to existing predictive atmospheric models for astronomical exposure time calculators.

Statistical Methods for Astronomy

astro-ph — Thu, 10 May 2012 00:38:41 +0000

This review outlines concepts of mathematical statistics, elements of probability theory, hypothesis tests and point estimation for use in the analysis of modern astronomical data. Least squares, maximum likelihood, and Bayesian approaches to statistical inference are treated. Resampling methods, particularly the bootstrap, provide valuable procedures when distributions functions of statistics are not known. Several approaches to model selection and good- ness of fit are considered. Applied statistics relevant to astronomical research are briefly discussed: nonparametric methods for use when little is known about the behavior of the astronomical populations or processes; data smoothing with kernel density estimation and nonparametric regression; unsupervised clustering and supervised classification procedures for multivariate problems; survival analysis for astronomical datasets with nondetections; time- and frequency-domain times series analysis for light curves; and spatial statistics to interpret the spatial distributions of points in low dimensions. Two types of resources are presented: about 40 recommended texts and monographs in various fields of statistics, and the public domain R software system for statistical analysis. Together with its \sim 3500 (and growing) add-on CRAN packages, R implements a vast range of statistical procedures in a coherent high-level language with advanced graphics.

A reconstruction method for neutrino induced muon tracks taking into account the apriori knowledge of the neutrino source [Replacement]

astro-ph — Thu, 10 May 2012 00:35:04 +0000

Gamma ray earthbound and satellite experiments have discovered, over the last years, many galactic and extra-galactic gamma ray sources. The detection of astrophysical neutrinos emitted by the same sources would imply that these astrophysical objects are charged cosmic ray accelerators and help to resolve the enigma of the origin of cosmic rays. A very large volume neutrino telescope might be able to detect these potential neutrino emitters. The apriori known direction of the neutrino source can be used to effectively suppress the $^{40}K$ optical background and increase significantly the tracking efficiency through causality filters. We report on advancing filtering and prefit techniques using the known neutrino source direction and first results are presented.

Reconstruction efficiency and discovery potential of a Mediterranean neutrino telescope: A simulation study using the Hellenic Open University Reconstruction & Simulation (HOURS) package [Replacement]

astro-ph — Thu, 10 May 2012 00:34:56 +0000

We report on the evaluation of the performance of a Mediterranean very large volume neutrino telescope. We present results of our studies concerning the capability of the telescope in detecting/discovering galactic (steady point sources) and extragalactic, transient (Gamma Ray Bursts) high energy neutrino sources as well as measuring ultra high energy diffuse neutrino fluxes. The neutrino effective area and angular resolution are presented as a function of the neutrino energy, and the background event rate (atmospheric neutrinos and muons) is estimated. The discovery potential of the neutrino telescope is evaluated and the experimental time required for a significant discovery of potential neutrino emitters (known from their gamma ray emission, assumedly produced by hadronic interactions) is estimated. For the simulation we use the HOU Reconstruction & Simulation (HOURS) software package.

A Feasibility Study for the Detection of Supernova Explosions with an Undersea Neutrino Telescope [Replacement]

astro-ph — Thu, 10 May 2012 00:34:47 +0000

We study the potential of a very large volume underwater Mediterranean neutrino telescope to observe neutrinos from supernova (SN) explosions within our galaxy. The intense neutrino burst emitted in a SN explosion results in a large number of MeV neutrinos inside the instrumented volume of the neutrino telescope that can be detected (mainly) via the reaction \nu_e-bar + p -> e^+ + n . In this study we simulated the response of the underwater neutrino telescope to the electron antineutrino flux predicted by the Garching model for SN explosions. We assumed that the neutrino telescope comprises 6160 direction sensitive optical modules, each containing 31 small photomultiplier tubes. Multiple coincidences between the photomultiplier tubes of the same optical module are utilized to suppress the noise produced by $^{40}K$ radioactive decays and to establish a statistical significant signature of the SN explosion.

WISeREP – An Interactive Supernova Data Repository [Replacement]

astro-ph — Thu, 10 May 2012 00:32:51 +0000

We have entered an era of massive data sets in astronomy. In particular, the number of supernova (SN) discoveries and classifications has substantially increased over the years from few tens to thousands per year. It is no longer the case that observations of a few prototypical events encapsulate most spectroscopic information about SNe, motivating the development of modern tools to collect, archive, organize and distribute spectra in general, and SN spectra in particular. For this reason we have developed the Weizmann Interactive Supernova data REPository – WISeREP – an SQL-based database (DB) with an interactive web-based graphical interface. The system serves as an archive of high quality SN spectra, including both historical (legacy) data as well as data that is accumulated by ongoing modern programs. The archive provides information about objects, their spectra, and related meta-data. Utilizing interactive plots, we provide a graphical interface to visualize data, perform line identification of the major relevant species, determine object redshifts, classify SNe and measure expansion velocities. Guest users may view and download spectra or other data that have been placed in the public domain. Registered users may also view and download data that are proprietary to specific programs with which they are associated. The DB currently holds >8000 spectra, of which >5000 are public; the latter include published spectra from the Palomar Transient Factory, all of the SUSPECT archive, the Caltech-Core-Collapse Program, the CfA SN spectra archive and published spectra from the UC Berkeley SNDB repository. It offers an efficient and convenient way to archive data and share it with colleagues, and we expect that data stored in this way will be easy to access, increasing its visibility, usefulness and scientific impact.

Nano dust impacts on spacecraft and boom antenna charging [Replacement]

astro-ph — Thu, 10 May 2012 00:31:32 +0000

High rate sampling detectors measuring the potential difference between the main body and boom antennas of interplanetary spacecraft have been shown to be efficient means to measure the voltage pulses induced by nano dust impacts on the spacecraft body itself (see Meyer-Vernet et al, Solar Phys. 256, 463 (2009)). However, rough estimates of the free charge liberated in post impact expanding plasma cloud indicate that the cloud’s own internal electrostatic field is too weak to account for measured pulses as the ones from the TDS instrument on the STEREO spacecraft frequently exceeding 0.1 V/m. In this paper we argue that the detected pulses are not a direct measure of the potential structure of the plasma cloud, but are rather the consequence of a transitional interruption of the photoelectron return current towards the portion of the antenna located within the expanding cloud.

On the unconstrained expansion of a spherical plasma cloud turning collisionless : case of a cloud generated by a nanometer dust grain impact on an uncharged target in space [Cross-Listing]

astro-ph — Wed, 09 May 2012 00:35:47 +0000

Nano and micro meter sized dust particles travelling through the heliosphere at several hundreds of km/s have been repeatedly detected by interplanetary spacecraft. When such fast moving dust particles hit a solid target in space, an expanding plasma cloud is formed through the vaporisation and ionisation of the dust particles itself and part of the target material at and near the impact point. Immediately after the impact the small and dense cloud is dominated by collisions and the expansion can be described by fluid equations. However, once the cloud has reached micro-m dimensions, the plasma may turn collisionless and a kinetic description is required to describe the subsequent expansion. In this paper we explore the late and possibly collisionless spherically symmetric unconstrained expansion of a single ionized ion-electron plasma using N-body simulations. Given the strong uncertainties concerning the early hydrodynamic expansion, we assume that at the time of the transition to the collisionless regime the cloud density and temperature are spatially uniform. We do also neglect the role of the ambient plasma. This is a reasonable assumption as long as the cloud density is substantially higher than the ambient plasma density. In the case of clouds generated by fast interplanetary dust grains hitting a solid target some 10^7 electrons and ions are liberated and the in vacuum approximation is acceptable up to meter order cloud dimensions. …

Measurement of scintillation efficiency for nuclear recoils in liquid argon [Replacement]

astro-ph — Wed, 09 May 2012 00:33:36 +0000

The scintillation light yield of liquid argon from nuclear recoils relative to electronic recoils has been measured as a function of recoil energy from 10 keVr up to 250 keVr. The scintillation efficiency, defined as the ratio of the nuclear recoil scintillation response to the electronic recoil response, is 0.25 \pm 0.01 + 0.01(correlated) above 20 keVr.

Gone with the Wind ON_Mars (GOWON): A Wind-Driven Networked System of Mobile Sensors on Mars [Replacement]

astro-ph — Wed, 09 May 2012 00:32:38 +0000

We propose a revolutionary way of studying the sur-face of Mars using a wind-driven network of mobile sensors- Gone with the Wind ON_Mars (GOWON). GOWON is envisioned to be a scalable, 100% self energy-generating and distributed system that allows in-situ mapping of a wide range of phenomena in a much larger portion of the surface of Mars compared to earlier missions. It could radically improve the possibility of finding rare phenomena like bio signatures through random wind-driven search. It could explore difficult terrains that were beyond the reach of previous missions, such as regions with very steep slopes, cluttered surfaces and/or sand dunes; GOWON is envisioned as an on going mission with a long life span. It could achieve any of NASA’s scientific objectives on Mars in a cost-effective way, leaving a long lasting sensing and searching infrastructure on Mars. GOWON is a 2012 Step B invitee for NASA Innovative Advanced Concept (NIAC). It addresses the challenge area of the Mars Surface System Capabilities area. We believe the challenge to be near-term, i.e., 2018-2024.

Numerical Simulations of Optically Thick Accretion onto a Black Hole – I. Spherical Case [Replacement]

astro-ph — Wed, 09 May 2012 00:32:14 +0000

Modeling the radiation generated by accreting matter is an important step towards realistic simulations of black hole accretion disks, especially at high accretion rates. To this end, we have recently added radiation transport to the existing general relativistic magnetohydrodynamic code, Cosmos++. However, before attempting to model radiative accretion disks, we have tested the new code using a series of shock tube and Bondi (spherical inflow) problems. The four radiative shock tube tests, first presented by Farris et al. (2008), have known analytic solutions, allowing us to calculate errors and convergence rates for our code. The Bondi problem only has an analytic solution when radiative processes are ignored, but is pertinent because it is closer to the physics we ultimately want to study. In our simulations, we include Thomson scattering and thermal bremsstrahlung in the opacity, focusing exclusively on the super-Eddington regime. Unlike accretion onto bodies with solid surfaces, super-Eddington accretion onto black holes does not produce super-Eddington luminosity. In our examples, despite accreting at up to 300 times the Eddington rate, our measured luminosity is always several orders of magnitude below Eddington.

Interactive Visualization and Simulation of Astronomical Nebulae [Replacement]

astro-ph — Wed, 09 May 2012 00:32:06 +0000

Nano dust impacts on spacecraft and boom antenna charging [Replacement]

astro-ph — Wed, 09 May 2012 00:31:13 +0000

High rate sampling detectors measuring the potential difference between the main body and boom antennas of interplanetary spacecraft have been shown to be efficient means to measure the voltage pulses induced by nano dust impacts on the spacecraft body itself (see Meyer-Vernet et al, Solar Phys. 256, 463 (2009)). However, rough estimates of the free charge liberated in post impact expanding plasma cloud indicate that the cloud’s own internal electrostatic field is too weak to account for measured pulses as the ones from the TDS instrument on the STEREO spacecraft frequently exceeding 0.1 V/m$. In this paper we argue that the detected pulses are not a direct measure of the potential structure of the plasma cloud, but are rather the consequence of a transitional interruption of the photoelectron return current towards the portion of the antenna located within the expanding cloud.

Accelerating NBODY6 with Graphics Processing Units

astro-ph — Tue, 08 May 2012 00:54:56 +0000

We describe the use of Graphics Processing Units (GPUs) for speeding up the code NBODY6 which is widely used for direct $N$-body simulations. Over the years, the $N^2$ nature of the direct force calculation has proved a barrier for extending the particle number. Following an early introduction of force polynomials and individual time-steps, the calculation cost was first reduced by the introduction of a neighbour scheme. After a decade of GRAPE computers which speeded up the force calculation further, we are now in the era of GPUs where relatively small hardware systems are highly cost-effective. A significant gain in efficiency is achieved by employing the GPU to obtain the so-called regular force which typically involves some 99 percent of the particles, while the remaining local forces are evaluated on the host. However, the latter operation is performed up to 20 times more frequently and may still account for a significant cost. This effort is reduced by parallel SSE/AVX procedures where each interaction term is calculated using mainly single precision. We also discuss further strategies connected with coordinate and velocity prediction required by the integration scheme. This leaves hard binaries and multiple close encounters which are treated by several regularization methods. The present nbody6-GPU code is well balanced for simulations in the particle range $10^4-2 \times 10^5$ for a dual GPU system attached to a standard PC.

Multi-color Cavity Metrology

astro-ph — Tue, 08 May 2012 00:53:38 +0000

Long baseline laser interferometers used for gravitational wave detection have proven to be very complicated to control. In order to have sufficient sensitivity to astrophysical gravitational waves, a set of multiple coupled optical cavities comprising the interferometer must be brought into resonance with the laser field. A set of multi-input, multi-output servos then lock these cavities into place via feedback control. This procedure, known as lock acquisition, has proven to be a vexing problem and has reduced greatly the reliability and duty factor of the past generation of laser interferometers. In this article, we describe a technique for bringing the interferometer from an uncontrolled state into resonance by using harmonically related external fields to provide a deterministic hierarchical control. This technique reduces the effect of the external seismic disturbances by four orders of magnitude and promises to greatly enhance the stability and reliability of the current generation of gravitational wave detector. The possibility for using multi-color techniques to overcome current quantum and thermal noise limits is also discussed.

Testing a hypothesis of the \nu Octantis planetary system

astro-ph — Tue, 08 May 2012 00:48:39 +0000

Design and Construction of Absorption Cells for Precision Radial Velocities in the K Band using Methane Isotopologues

astro-ph — Tue, 08 May 2012 00:45:21 +0000

We present a method to optimize absorption cells for precise wavelength calibration in the near-infrared. We apply it to design and optimize methane isotopologue cells for precision radial velocity measurements in the K band. We also describe the construction and installation of two such cells for the CSHELL spectrograph at NASA’s IRTF. We have obtained their high-resolution laboratory spectra, which we can then use in precision radial velocity measurements and which can also have other applications. In terms of obtainable RV precision methane should out-perform other proposed cells, such as the ammonia cell ($^{14}$NH$_{3}$) recently demonstrated on CRIRES/VLT. The laboratory spectra of Ammonia and the Methane cells show strong absorption features in the H band that could also be exploited for precision Doppler measurements. We present spectra and preliminary radial velocity measurements obtained during our first-light run. These initial results show that a precision down to 20-30 m s$^{-1}$ can be obtained using a wavelength interval of only 5 nm in the K band and S/N$\sim$150. This supports the prediction that a precision down to a few m s$^{-1}$ can be achieved on late M dwarfs using the new generation of NIR spectrographs, thus enabling the detection of terrestrial planets in their habitable zones. Doppler measurements in the NIR can also be used to mitigate the radial velocity jitter due to stellar activity enabling more efficient surveys on young active stars.

Redundancy Calibration of Phased Array Stations

astro-ph — Tue, 08 May 2012 00:43:38 +0000

Our aim is to assess the benefits and limitations of using the redundant visibility information in regular phased array systems for improving the calibration. Regular arrays offer the possibility to use redundant visibility information to constrain the calibration of the array independent of a sky model and a beam models of the station elements. It requires a regular arrangement in the configuration of array elements and identical beam patterns. We revised a calibration method for phased array stations using the redundant visibility information in the system and applied it successfully to a LOFAR station. The performance and limitations of the method were demonstrated by comparing its use on real and simulated data. The main limitation is the mutual coupling between the station elements, which leads to non-identical beams and stronger baseline dependent noise. Comparing the variance of the estimated complex gains with the Cramer-Rao Bound (CRB) indicates that redundancy is a stable and optimum method for calibrating the complex gains of the system. Our study shows that the use of the redundant visibility does improve the quality of the calibration in phased array systems. In addition it provides a powerful tool for system diagnostics. Our results demonstrate that designing redundancy in both the station layout and the array configuration of future aperture arrays is strongly recommended. In particular in the case of the Square Kilometre Array with its dynamic range requirement which surpasses any existing array by an order of magnitude.

Nano dust impacts on spacecraft and boom antenna charging

astro-ph — Tue, 08 May 2012 00:43:25 +0000

High rate sampling detectors measuring the potential difference between the main body and boom antennas of interplanetary spacecraft have been shown to be efficient means to measure the voltage pulses induced by nano dust impacts on the spacecraft body itself (see Meyer-Vernet et al, Solar Phys. 256, 463 (2009)). However, rough estimates of the free charge liberated in post impact expanding plasma cloud indicate that the cloud’s own internal electrostatic field is too weak to account for measured pulses as the ones from the TDS instrument on the STEREO spacecraft frequently exceeding 0.1 V/m$. In this paper we argue that the detected pulses are not a direct measure of the potential structure of the plasma cloud, but are rather the consequence of a transitional interruption of the photoelectron return current towards the portion of the antenna located within the expanding cloud.

AGN Physics with the Cherenkov Telescope Array

astro-ph — Tue, 08 May 2012 00:41:12 +0000

Possible direct measurement of the expansion rate of the universe [Replacement]

astro-ph — Tue, 08 May 2012 00:39:26 +0000

A new method is proposed for directly measuring the expansion rate of the universe through very precise measurement of the fluence of extremely stable sources. The method is based on the definition of the luminosity distance and its change along the time due to the cosmic expansion. It is argued that galaxies may be chosen as the targets of the observation to perform the measurement. We show that, by simultaneously increasing the observation time and physically adding the fluences from different galaxies, the requirement on the relative precision of the detector for an observation of 1 second on a single galaxy can be relaxed to $10^{-5}$. Benefiting from the abundance of galaxies in the universe, the method may be quite promising.

A data-driven model for spectra: Finding double redshifts in the Sloan Digital Sky Survey [Replacement]

astro-ph — Tue, 08 May 2012 00:36:03 +0000

We present a data-driven method – heteroscedastic matrix factorization, a kind of probabilistic factor analysis – for modeling or performing dimensionality reduction on observed spectra or other high-dimensional data with known but non-uniform observational uncertainties. The method uses an iterative inverse-variance-weighted least-squares minimization procedure to generate a best set of basis functions. The method is similar to principal components analysis, but with the substantial advantage that it uses measurement uncertainties in a responsible way and accounts naturally for poorly measured and missing data; it models the variance in the noise-deconvolved data space. A regularization can be applied, in the form of a smoothness prior (inspired by Gaussian processes) or a non-negative constraint, without making the method prohibitively slow. Because the method optimizes a justified scalar (related to the likelihood), the basis provides a better fit to the data in a probabilistic sense than any PCA basis. We test the method on SDSS spectra, concentrating on spectra known to contain two redshift components: These are spectra of gravitational lens candidates and massive black-hole binaries. We apply a hypothesis test to compare one-redshift and two-redshift models for these spectra, utilizing the data-driven model trained on a random subset of all SDSS spectra. This test confirms 129 of the 131 lens candidates in our sample and all of the known binary candidates, and turns up very few false positives.

Comparison of Fermi-LAT and CTA in the region between 10-100 GeV

astro-ph — Mon, 07 May 2012 00:49:06 +0000

The past decade has seen a dramatic improvement in the quality of data available at both high (HE: 100 MeV to 100 GeV) and very high (VHE: 100 GeV to 100 TeV) gamma-ray energies. With three years of data from the Fermi Large Area Telescope (LAT) and deep pointed observations with arrays of Cherenkov telescope, continuous spectral coverage from 100 MeV to $\sim10$ TeV exists for the first time for the brightest gamma-ray sources. The Fermi-LAT is likely to continue for several years, resulting in significant improvements in high energy sensitivity. On the same timescale, the Cherenkov Telescope Array (CTA) will be constructed providing unprecedented VHE capabilities. The optimisation of CTA must take into account competition and complementarity with Fermi, in particularly in the overlapping energy range 10$-$100 GeV. Here we compare the performance of Fermi-LAT and the current baseline CTA design for steady and transient, point-like and extended sources.

Low energy electron/recoil discrimination for directional Dark Matter detection

astro-ph — Mon, 07 May 2012 00:41:44 +0000

Directional detection is a promising Dark Matter search strategy. Even though it could accommodate to a sizeable background contamination, electron/recoil discrimination remains a key and challenging issue as for direction-insensitive detectors. The measurement of the 3D track may be used to discriminate electrons from nuclear recoils. While a high rejection power is expected above 20 keV ionization, a dedicated data analysis is needed at low energy. After identifying discriminant observables, a multivariate analysis, namely a Boosted Decision Tree, is proposed, enabling an efficient event tagging for Dark Matter search. We show that it allows us to optimize rejection while keeping a rather high efficiency which is compulsory for rare event search.With respect to a sequential analysis, the rejection is about 20 times higher with a multivariate analysis, for the same Dark Matter exclusion limit.

Sparse signal reconstruction on the sphere: implications of a new sampling theorem [Cross-Listing]

astro-ph — Mon, 07 May 2012 00:40:37 +0000

A new sampling theorem on the sphere has been developed recently, reducing the number of samples required to represent a band-limited signal by a factor of two for equiangular sampling schemes. For signals sparse in a spatially localised measure, such as in a wavelet basis, overcomplete dictionary, or in the magnitude of their gradient, for example, a reduction in the number of samples required to represent a band-limited signal has important implications for sparse signal reconstruction on the sphere. A more efficient sampling of the sphere improves the fidelity of sparse signal reconstruction through both the dimensionality and spatial sparsity of signals. To demonstrate this result we consider a simple inpainting problem on the sphere and consider signals sparse in the magnitude of their gradient. We develop a framework for total variation (TV) inpainting on the sphere by making a connection to the underlying continuous signal via a sampling theorem. Numerical simulations are performed, verifying the enhanced fidelity of sparse signal reconstruction due to the more efficient sampling of the sphere provided by the new sampling theorem.

Impact of Redshift Information on Cosmological Applications with Next-Generation Radio Surveys

astro-ph — Mon, 07 May 2012 00:38:22 +0000

In this paper we explore the impact of including redshift information on cosmological applications with the forthcoming generation of large-scale, deep radio continuum surveys. By cross-matching these radio surveys with shallow optical to near-infrared surveys we can essentially separate the source distribution into a low redshift sample and the high-z tail of the radio sources that are unidentified, thus providing a constraint on the evolution of cosmological parameters such as those of dark energy. We examine two radio surveys, the Evolutionary Map of the Universe (EMU) and the Westerbork Observations of the Deep APERTIF Northern sky (WODAN). A crucial advantage is their combined potential to provide a deep, full-sky survey. The surveys used for the cross-identifications are SkyMapper and SDSS, for the southern and northern skies, respectively. We concentrate on the galaxy clustering angular power spectrum as our benchmark observable and find that the possibility of including this low redshift information yields major improvements in the results. With this approach, we are able to put strict constraints on the dark energy parameters, i.e. w_0=-0.9+/-0.065(0.087) and w_a=-0.24+/-0.19(0.26) with(without) priors from Planck; this corresponds to a Figure of Merit (FoM) of circa 400(>200), which is two to three orders of magnitude times better than the case without any redshift information and more than three times better than what obtained by using only the cross-identified sources.

Early Science with SOFIA, the Stratospheric Observatory for Infrared Astronomy

astro-ph — Fri, 04 May 2012 00:42:09 +0000

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is an airborne observatory consisting of a specially modified Boeing 747SP with a 2.7-m telescope, flying at altitudes as high as 13.7 km (45,000 ft). Designed to observe at wavelengths from 0.3 micron to 1.6 mm, SOFIA operates above 99.8 % of the water vapor that obscures much of the infrared and submillimeter. SOFIA has seven science instruments under development, including an occultation photometer, near-, mid-, and far-infrared cameras, infrared spectrometers, and heterodyne receivers. SOFIA, a joint project between NASA and the German Aerospace Center DLR, began initial science flights in 2010 December, and has conducted 30 science flights in the subsequent year. During this early science period three instruments have flown: the mid-infrared camera FORCAST, the heterodyne spectrometer GREAT, and the occultation photometer HIPO. This article provides an overview of the observatory and its early performance.

Exact Wavelets on the Ball [Cross-Listing]

astro-ph — Fri, 04 May 2012 00:41:52 +0000

We develop an exact wavelet transform on the three-dimensional ball (i.e. on the solid sphere), which we name the flaglet transform. For this purpose we first construct an exact harmonic transform on the radial line using damped Laguerre polynomials and develop a corresponding quadrature rule. Combined with the spherical harmonic transform, this approach leads to a sampling theorem on the ball and a novel three-dimensional decomposition which we call the Fourier-Laguerre transform. We relate this new transform to the well-known Fourier-Bessel decomposition and show that band-limitness in the Fourier-Laguerre basis is a sufficient condition to compute the Fourier-Bessel decomposition exactly. We then construct the flaglet transform on the ball through a harmonic tiling, which is exact thanks to the exactness of the Fourier-Laguerre transform (from which the name flaglets is coined). The corresponding wavelet kernels have compact localisation properties in real and harmonic space and their angular aperture is invariant under radial translation. We introduce a multiresolution algorithm to perform the flaglet transform rapidly, while capturing all information at each wavelet scale in the minimal number of samples on the ball. Our implementation of these new tools achieves floating point precision and is made publicly available. We perform numerical experiments demonstrating the speed and accuracy of these libraries and illustrate their capabilities on a simple denoising example.

A Sensitivity and Array-Configuration Study for Measuring the Power Spectrum of 21cm Emission from Reionization [Replacement]

astro-ph — Fri, 04 May 2012 00:40:04 +0000

Telescopes aiming to measure 21cm emission from the Epoch of Reionization must toe a careful line, balancing the need for raw sensitivity against the stringent calibration requirements for removing bright foregrounds. It is unclear what the optimal design is for achieving both of these goals. Via a pedagogical derivation of an interferometer’s response to the power spectrum of 21cm reionization fluctuations, we show that even under optimistic scenarios, first-generation arrays will yield low-SNR detections, and that different compact array configurations can substantially alter sensitivity. We explore the sensitivity gains of array configurations that yield high redundancy in the uv-plane — configurations that have been largely ignored since the advent of self-calibration for high-dynamic-range imaging. We first introduce a mathematical framework to generate optimal minimum-redundancy configurations for imaging. We contrast the sensitivity of such configurations with high-redundancy configurations, finding that high-redundancy configurations can improve power-spectrum sensitivity by more than an order of magnitude. We explore how high-redundancy array configurations can be tuned to various angular scales, enabling array sensitivity to be directed away from regions of the uv-plane (such as the origin) where foregrounds are brighter and where instrumental systematics are more problematic. We demonstrate that a 132-antenna deployment of the Precision Array for Probing the Epoch of Reionization (PAPER) observing for 120 days in a high-redundancy configuration will, under ideal conditions, have the requisite sensitivity to detect the power spectrum of the 21cm signal from reionization at a 3\sigma level at k<0.25h Mpc^{-1} in a bin of \Delta ln k=1. We discuss the tradeoffs of low- versus high-redundancy configurations.

A way to deal with the fringe-like pattern in VIMOS-IFU data [Replacement]

astro-ph — Fri, 04 May 2012 00:33:45 +0000

The use of integral field units is now commonplace at all major observatories offering efficient means of obtaining spectral as well as imaging information at the same time. IFU instrument designs are complex and spectral images have typically highly condensed formats, therefore presenting challenges for the IFU data reduction pipelines. In the case of the VLT VIMOS-IFU, a fringe-like pattern affecting the spectra well into the optical and blue wavelength regime as well as artificial intensity variations, require additional reduction steps beyond standard pipeline processing. In this research note we propose an empirical method for the removal of the fringe-like pattern in the spectral domain and the intensity variations in the imaging domain. We also demonstrate the potential consequences for data analysis if the effects are not corrected. Here we use the example of deriving stellar velocity, velocity dispersion and absorption line-strength maps for early-type galaxies. We derive for each spectrum, reduced by the ESO standard VIMOS pipeline, a correction-spectrum by using the median of the eight surrounding spectra as a proxy for the unaffected, underlying spectrum. This method relies on the fact that our science targets (nearby ETGs) cover the complete FoV of the VIMOS-IFU with slowly varying spectral properties and that the exact shape of the fringe-like pattern is nearly independent and highly variable between neighboring spatial positions. We find that the proposed correction methods for the removal of the fringe-like pattern and the intensity variations in VIMOS-IFU data-cubes are suitable to allow for meaningful data analysis in our sample of nearby early-type galaxies. Since the method relies on the scientific target properties it is not suitable for general implementation in the pipeline software for VIMOS.

A Distributed GPU-based Framework for real-time 3D Volume Rendering of Large Astronomical Data Cubes

astro-ph — Thu, 03 May 2012 00:45:08 +0000

We present a framework to interactively volume-render three-dimensional data cubes using distributed ray-casting and volume bricking over a cluster of workstations powered by one or more graphics processing units (GPUs) and a multi-core CPU. The main design target for this framework is to provide an in-core visualization solution able to provide three-dimensional interactive views of terabyte-sized data cubes. We tested the presented framework using a computing cluster comprising 64 nodes with a total of 128 GPUs. The framework proved to be scalable to render a 204 GB data cube with an average of 30 frames per second. Our performance analyses also compare between using NVIDIA Tesla 1060 and 2050 GPU architectures and the effect of increasing the visualization output resolution on the rendering performance. Although our initial focus, and the examples presented in this work, is volume rendering of spectral data cubes from radio astronomy, we contend that our approach has applicability to other disciplines where close to real-time volume rendering of terabyte-order 3D data sets is a requirement.

Effects of Thomson-Scattering Geometry on White-Light Imaging of an Interplanetary Shock: Synthetic Observations from Forward Magnetohydrodynamic Modelling

astro-ph — Thu, 03 May 2012 00:43:38 +0000

Stereoscopic white-light imaging of a large portion of the inner heliosphere has been used to track interplanetary coronal mass ejections. At large elongations from the Sun, the white-light brightness depends on both the local electron density and the efficiency of the Thomson-scattering process. To quantify the effects of the Thomson-scattering geometry, we study an interplanetary shock using forward magnetohydrodynamic simulation and synthetic white-light imaging. Identifiable as an inclined streak of enhanced brightness in a time-elongation map, the travelling shock can be readily imaged by an observer located within a wide range of longitudes in the ecliptic. Different parts of the shock front contribute to the imaged brightness pattern viewed by observers at different longitudes. Moreover, even for an observer located at a fixed longitude, a different part of the shock front will contribute to the imaged brightness at any given time. The observed brightness within each imaging pixel results from a weighted integral along its corresponding ray-path. It is possible to infer the longitudinal location of the shock from the brightness pattern in an optical sky map, based on the east-west asymmetry in its brightness and degree of polarization. Therefore, measurement of the interplanetary polarized brightness could significantly reduce the ambiguity in performing three-dimensional reconstruction of local electron density from white-light imaging.

Evaluation of the Neutron Background in a Direct WIMP Detector with Germanium Target using Reactor Neutrino Detector as Neutron Veto

astro-ph — Thu, 03 May 2012 00:39:37 +0000

A direct WIMP (Weakly Interacting Massive Particle) detector with a neutron veto system is designed to better reject neutrons. An experimental configuration is studied in the present paper: 984 Ge modules are placed inside a reactor neutrino detector. The neutrino detector is used as a neutron veto device. The neutron background for the experimental design has been estimated using the Geant4 simulation. The result show that the neutron background can decrease to O(0.01) events per year per tonne of high purity Germanium. We calculate the sensitivity to spin-independent WIMP-nucleon elastic scattering. An exposure of one tonne $\times$ year could reach a cross-section of about 4$\times$$10^{-11}$ pb.

Advances in understanding young high-mass stars using optical interferometry

astro-ph — Thu, 03 May 2012 00:38:23 +0000

The closest examples of high-mass star birth occurs in deeply embedded environments at kiloparsec distances. Although much progress has been made, an observationally validated picture of the dominant processes which allows the central hydrostatic object to grow in mass has yet to be established. The observational technique of optical interferometry has demonstrated its potential in the field of high-mass star formation by delivering a milli-arcsecond infrared view on the complex accretion environment. We provide an overview of the scientific results obtained with multi-aperture telescope arrays and briefly discuss future instruments and their anticipated impact on our understanding of massive young stellar objects.

Recognizing magnetic structures by present and future radio telescopes with RM Synthesis [Replacement]

astro-ph — Thu, 03 May 2012 00:32:52 +0000

We investigate the possibilities of wavelet-based RM Synthesis for the recognition of structures of regular and turbulent magnetic fields in extended magnetized objects, like galaxies and galaxy clusters. Wavelets allow to reformulate the RM Synthesis method in a scale-dependent way and to visualize the data as a function of Faraday depth and scale. We present observational tests to recognize regular magnetic fields without and with one or two reversals along the line of sight and imprints of turbulent magnetic fields. A region with a regular magnetic field generates a broad “disk” in Faraday space (“Faraday spectrum”), with two “horns” if the distribution of cosmic-ray electrons is broader than that of the thermal electrons. Each field reversal generates one asymmetric “horn” on top of the “disk”. A region with a turbulent field can be recognized by a “Faraday forest” of many components. We argue that the ratio of maximum to minimum wavelengths is an important parameter because it determines the range of scales which can be identified in Faraday space. Full recognition of magnetic field structures in spiral galaxies or galaxy clusters requires analysis of data cubes in position-position-Faraday depth (“PPF cubes”), observed over a wide and continuous wavelength range, from about 100 MHz to several GHz, providing good resolution as well as recognition of a wide range of scales in Faraday space. The planned SKA fulfils this condition and will be close to a perfect “Faraday telescope”. The combination of data from the present telescopes LOFAR (low frequencies) and EVLA (high frequencies) looks promising for the recognition of magnetic structures at all scales. The addition of WSRT or GMRT data at intermediate frequencies to those from LOFAR and EVLA data fills the gap between the LOFAR and EVLA wavelength ranges.

First Large Scale Production of Low Radioactivity Argon From Underground Sources [Replacement]

astro-ph — Thu, 03 May 2012 00:32:17 +0000

We report on the first large-scale production of low radioactivity argon from underground gas wells. Low radioactivity argon is of general interest, in particular for the construction of large scale WIMP dark matter searches and detectors of reactor neutrinos for non-proliferation efforts. Atmospheric argon has an activity of about 1 Bq/kg from the decays of 39Ar; the concentration of 39Ar in the underground argon we are collecting is at least a factor of 100 lower than this value. The argon is collected from a stream of gas from a CO2 well in southwestern Colorado with a Vacuum Pressure Swing Adsorption (VPSA) plant. The gas from the well contains argon at a concentration of 400-600 ppm, and the VPSA plant produces an output stream with an argon concentration at the level of 30,000-50,000 ppm (3-5%) in a single pass. This gas is sent for further processing to Fermilab where it is purified by cryogenic distillation. The argon production rate is presently 0.5 kg/day.

First Commissioning of a Cryogenic Distillation Column for Low Radioactivity Underground Argon [Replacement]

astro-ph — Thu, 03 May 2012 00:31:52 +0000

We report on the performance and commissioning of a cryogenic distillation column for low radioactivity underground argon at Fermi National Accelerator Laboratory. The distillation column is designed to accept a mixture of argon, helium, and nitrogen and return pure argon with a nitrogen contamination less than 10 ppm. In the first commissioning, we were able to run the distillation column in a continuous mode and produce argon that is 99.9% pure. After running in a batch mode, the argon purity was increased to 99.95%, with 500 ppm of nitrogen remaining. The efficiency of collecting the argon from the gas mixture was between 70% and 81%, at an argon production rate of 0.84-0.98 kg/day.

EzGal: A Flexible Interface for Stellar Population Synthesis Models

astro-ph — Wed, 02 May 2012 00:48:13 +0000

We present EzGal, a flexible python program designed to easily generate observable parameters (magnitudes, colors, mass-to-light ratios) for any stellar population synthesis (SPS) model. As has been demonstrated by various authors, the choice of input SPS models can be a significant source of systematic uncertainty. A key strength of EzGal is that it enables simple, direct comparison of different models sets. EzGal is also capable of generating composite stellar population models (CSPs) and can interpolate between metallicities for a given model set. We have created a web interface to run EzGal and generate observables for a variety of star formation histories and model sets. We make many commonly used SPS models available from this interface; the BC03 models, an updated version of these models, the Maraston models, the BaSTI models, and finally the FSPS models. We use EzGal to compare magnitude predictions for the model sets as a function of wavelength, age, metallicity, and star formation history. We recover the well-known result that the models agree best in the optical for old, solar metallicity models, with differences at the ~0.1 magnitude level. The most problematic regime for SPS modeling is for young ages (7500 Angstroms) where scatter between models can vary from 0.3 mags (Sloan i’) to 0.7 mags (Ks). We find that these differences are best understood as general uncertainties in SPS modeling. Finally we explore a more physically motivated example by generating CSPs with a star formation history matching the global star formation history of the universe. We demonstrate that the wavelength and age dependence of SPS model uncertainty translates into a redshift dependent model uncertainty, highlighting the importance of a quantitative understanding of model differences when comparing observations to models as a function of redshift.

Ross Ice Shelf in situ radio-frequency ice attenuation [Replacement]

astro-ph — Wed, 02 May 2012 00:37:30 +0000

We have measured the in situ average electric field attenuation length for radio-frequency signals broadcast vertically through the Ross Ice Shelf. We chose a location, Moore Embayment, south of Minna Bluff, known for its high reflectivity at the ice-sea interface. We confirmed specular reflection and used the return pulses to measure the average attenuation length from 75-1250 MHz over the round-trip distance of 1155 m. We find the average electric field attenuation length to vary from 500 m at 75 MHz to 300 m at 1250 MHz, with an experimental uncertainty of 55 to 15 m. We discuss the implications for neutrino telescopes that use the radio technique and include the Ross Ice Shelf as part of their sensitive volume.

Observation of a Free-Shercliff-Layer Instability in Cylindrical Geometry [Cross-Listing]

astro-ph — Tue, 01 May 2012 00:57:07 +0000

We report on observations of a free-Shercliff-layer instability in a Taylor-Couette experiment using a liquid metal over a wide range of Reynolds numbers, $Re\sim 10^3-10^6$. The free Shercliff layer is formed by imposing a sufficiently strong axial magnetic field across a pair of differentially rotating axial endcap rings. This layer is destabilized by a hydrodynamic Kelvin-Helmholtz-type instability, characterized by velocity fluctuations in the $r-\theta$ plane. The instability appears with an Elsasser number above unity, and saturates with an azimuthal mode number $m$ which increases with the Elsasser number. Measurements of the structure agree well with 2D global linear mode analyses and 3D global nonlinear simulations. These observations have implications for a range of rotating MHD systems in which similar shear layers may be produced.

Selecting Quasar Candidates by a SVM Classification System

astro-ph — Tue, 01 May 2012 00:56:48 +0000

We develop and demonstrate a classification system constituted by several Support Vector Machines (SVM) classifiers, which can be applied to select quasar candidates from large sky survey projects, such as SDSS, UKIDSS, GALEX. How to construct this SVM classification system is presented in detail. When the SVM classification system works on the test set to predict quasar candidates, it acquires the efficiency of 93.21% and the completeness of 97.49%. In order to further prove the reliability and feasibility of this system, two chunks are randomly chosen to compare its performance with that of the XDQSO method used for SDSS-III’s BOSS. The experimental results show that the high faction of overlap exists between the quasar candidates selected by this system and those extracted by the XDQSO technique in the dereddened i-band magnitude range between 17.75 and 22.45, especially in the interval of dereddened i-band magnitude < 20.0. In the two test areas, 57.38% and 87.15% of the quasar candidates predicted by the system are also targeted by the XDQSO method. Similarly, the prediction of subcategories of quasars according to redshift achieves a high level of overlap with these two approaches. Depending on the effectiveness of this system, the SVM classification system can be used to create the input catalog of quasars for the GuoShouJing Telescope (LAMOST) or other spectroscopic sky survey projects. In order to get higher confidence of quasar candidates, cross-result from the candidates selected by this SVM system with that by XDQSO method is applicable.

Application of GPUs for the Calculation of Two Point Correlation Functions in Cosmology

astro-ph — Tue, 01 May 2012 00:47:42 +0000

In this work, we have explored the advantages and drawbacks of using GPUs instead of CPUs in the calculation of a standard 2-point correlation function algorithm, which is useful for the analysis of Large Scale Structure of galaxies. Taking into account the huge volume of data foreseen in upcoming surveys, our main goal has been to accelerate significantly the analysis codes. We find that GPUs offer a 100-fold increase in speed with respect to a single CPU without a significant deviation in the results. For comparison’s sake, an MPI version was developed as well. Some issues, like code implementation, which arise from using this option are discussed.

Complexity reduction of astrochemical networks

astro-ph — Tue, 01 May 2012 00:46:07 +0000

We present a new computational scheme aimed at reducing the complexity of the chemical networks in astrophysical models, one which is shown to markedly improve their computational efficiency. It contains a flux-reduction scheme that permits to deal with both large and small systems. This procedure is shown to yield a large speed-up of the corresponding numerical codes and provides good accord with the full network results. We analyse and discuss two examples involving chemistry networks of the interstellar medium and show that the results from the present reduction technique reproduce very well the results from fuller calculations.

Adaptable Radiative Transfer Innovations for Submillimetre Telescopes (ARTIST) – Dust polarisation module (DustPol)

astro-ph — Tue, 01 May 2012 00:45:42 +0000

We present a new publicly available tool (DustPol) aimed to model the polarised thermal dust emission. The module DustPol, which is publicly available, is part of the ARTIST (Adaptable Radiative Transfer Innovations for Submillimetre Telescopes) package, which also offers tools for modelling the polarisation of line emission together with a model library and a Python-based user interface. DustPol can easily manage analytical as well as pre-gridded models to generate synthetic maps of the Stokes I, Q, and U parameters. These maps are stored in FITS format which is straightforwardly read by the data reduction software used, e.g., by the Atacama Large Millimeter Array (ALMA). This turns DustPol into a powerful engine for the prediction of the expected polarisation features of a source observed with ALMA or the Planck satellite as well as for the interpretation of existing submillimetre observations obtained with other telescopes. DustPol allows the parameterisation of the maximum degree of polarisation and we find that, in a prestellar core, if there is depolarisation, this effect should happen at densities of 10^6 cm-3 or larger. We compare a model generated by DustPol with the observational polarisation data of the low-mass Class 0 object NGC 1333 IRAS 4A, finding that the total and the polarised emission are consistent.

Fully Digital: Policy and Process Implications for the AAS

astro-ph — Tue, 01 May 2012 00:43:14 +0000

Over the past two decades, every scholarly publisher has migrated at least the mechanical aspects of their journal publishing so that they utilize digital means. The academy was comfortable with that for a while, but publishers are under increasing pressure to adapt further. At the American Astronomical Society (AAS), we think that means bringing our publishing program to the point of being fully digital, by establishing procedures and policies that regard the digital objects of publication primarily. We have always thought about our electronic journals as databases of digital articles, from which we can publish and syndicate articles one at a time, and we must now put flesh on those bones by developing practices that are consistent with the realities of article at a time publication online. As a learned society that holds the long-term rights to the literature, we have actively taken responsibility for the preservation of the digital assets that constitute our journals, and in so doing we have not forsaken the legacy pre-digital assets. All of us who serve as the long-term stewards of scholarship must begin to evolve into fully digital publishers.

PyCOOL – a Cosmological Object-Oriented Lattice code written in Python [Replacement]

astro-ph — Tue, 01 May 2012 00:36:31 +0000

There are a number of different phenomena in the early universe that have to be studied numerically with lattice simulations. This paper presents a graphics processing unit (GPU) accelerated Python program called PyCOOL that solves the evolution of scalar fields in a lattice with very precise symplectic integrators. The program has been written with the intention to hit a sweet spot of speed, accuracy and user friendliness. This has been achieved by using the Python language with the PyCUDA interface to make a program that is easy to adapt to different scalar field models. In this paper we derive the symplectic dynamics that govern the evolution of the system and then present the implementation of the program in Python and PyCUDA. The functionality of the program is tested in a chaotic inflation preheating model, a single field oscillon case and in a supersymmetric curvaton model which leads to Q-ball production. We have also compared the performance of a consumer graphics card to a professional Tesla compute card in these simulations. We find that the program is not only accurate but also very fast. To further increase the usefulness of the program we have equipped it with numerous post-processing functions that provide useful information about the cosmological model. These include various spectra and statistics of the fields. The program can be additionally used to calculate the generated curvature perturbation. The program is publicly available under GNU General Public License at https://github.com/jtksai/PyCOOL . Some additional information can be found from http://www.physics.utu.fi/tiedostot/theory/particlecosmology/pycool/ .

First Light LBT AO Images of HR 8799 bcde at 1.65 and 3.3 Microns: New Discrepancies between Young Planets and Old Brown Dwarfs [Replacement]

astro-ph — Tue, 01 May 2012 00:34:34 +0000

As the only directly imaged multiple planet system, HR 8799 provides a unique opportunity to study the physical properties of several planets in parallel. In this paper, we image all four of the HR 8799 planets at H-band and 3.3 microns with the new LBT adaptive optics system, PISCES, and LBTI/LMIRCam. Our images offer an unprecedented view of the system, allowing us to obtain H and 3.3$ micron photometry of the innermost planet (for the first time) and put strong upper-limits on the presence of a hypothetical fifth companion. We find that all four planets are unexpectedly bright at 3.3 microns compared to the equilibrium chemistry models used for field brown dwarfs, which predict that planets should be faint at 3.3 microns due to CH4 opacity. We attempt to model the planets with thick-cloudy, non-equilibrium chemistry atmospheres, but find that removing CH4 to fit the 3.3 micron photometry increases the predicted L’ (3.8 microns) flux enough that it is inconsistent with observations. In an effort to fit the SED of the HR 8799 planets, we construct mixtures of cloudy atmospheres, which are intended to represent planets covered by clouds of varying opacity. In this scenario, regions with low opacity look hot and bright, while regions with high opacity look faint, similar to the patchy cloud structures on Jupiter and L/T transition brown-dwarfs. Our mixed cloud models reproduce all of the available data, but self-consistent models are still necessary to demonstrate their viability.

Estimating column density from ammonia (1,1) emission in star-forming regions

astro-ph — Mon, 30 Apr 2012 01:03:35 +0000

We present a new, approximate method of calculating the column density of ammonia in mapping observations of the 23 GHz inversion lines. The temperature regime typically found in star forming regions allows for the assumption of a slowly varying partition function for ammonia. It is therefore possible to determine the column density using only the (J=1,K=1) inversion transition rather than the typical combination of the (1,1) and (2,2) transitions, with additional uncertainties comparable to or less than typical observational error. The proposed method allows column density and mass estimates to be extended into areas of lower signal to noise ratio. We show examples of column density maps around a number of cores in the W3 and Perseus star-forming regions made using this approximation, along with a comparison to the corresponding results obtained using the full two-transition approach. We suggest that this method is a useful tool in studying the distribution of mass around YSOs, particularly in the outskirts of the protostellar envelope where the (2,2) ammonia line is often undetectable on the short timescales necessary for large area mapping.

First Commissioning of a Cryogenic Distillation Column for Low Radioactivity Underground Argon

astro-ph — Mon, 30 Apr 2012 00:54:40 +0000

The search for habitable worlds: 1. The viability of a starshade mission

astro-ph — Mon, 30 Apr 2012 00:54:11 +0000

As part of NASA’s mission to explore habitable planets orbiting nearby stars, this paper explores the detection and characterization capabilities of a 4-m space telescope plus 50-m starshade located at the Earth-Sun L2 point, a.k.a. the New Worlds Observer (NWO). Our calculations include the true spectral types and distribution of stars on the sky, an iterative target selection protocol designed to maximize efficiency based on prior detections, and realistic mission constraints. We carry out both analytical calculations and simulated observing runs for a wide range in exozodiacal background levels ({\epsilon} = 1 – 100 times the local zodi brightness) and overall prevalence of Earth-like terrestrial planets ({\eta}\oplus = 0.1 – 1). We find that even without any return visits, the NWO baseline architecture (IWA = 65 mas, limiting FPB = 4\times10-11) can achieve a 95% probability of detecting and spectrally characterizing at least one habitable Earth-like planet, and an expectation value of ~3 planets found, within the mission lifetime and {\Delta}V budgets, even in the worst-case scenario ({\eta}\oplus = 0.1 and {\epsilon} = 100 zodis for every target). This achievement requires about one year of integration time spread over the 5 year mission, leaving the remainder of the telescope time for UV-NIR General Astrophysics. Cost and technical feasibility considerations point to a “sweet spot” in starshade design near a 50-m starshade effective diameter, with 12 or 16 petals, at a distance of 70,000-100,000 km from the telescope.

Optimal Interpolation and Prediction in Pulsar Timing

astro-ph — Mon, 30 Apr 2012 00:51:50 +0000

For pulsar projects it is often necessary to predict the pulse phase in advance, for example, when preparing for new observations. Interpolation of the pulse phase between existing measurements is also often required, for example, when folding X-ray or gamma-ray observations according to the radio pulse phase. Until now these procedures have been done using various ad hoc methods. The purpose of this paper is to show how to interpolate or predict the pulse phase optimally using statistical models of the various noise processes and the phase measurement uncertainty.

Interactive Visualization and Simulation of Astronomical Nebulae

astro-ph — Mon, 30 Apr 2012 00:50:57 +0000

The scaler mode in the Pierre Auger Observatory to study heliospheric modulation of cosmic rays

astro-ph — Mon, 30 Apr 2012 00:47:49 +0000

The impact of the solar activity on the heliosphere has a strong influence on the modulation of the flux of low energy galactic cosmic rays arriving at Earth. Different instruments, such as neutron monitors or muon detectors, have been recording the variability of the cosmic ray flux at ground level for several decades. Although the Pierre Auger Observatory was designed to observe cosmic rays at the highest energies, it also records the count rates of low energy secondary particles (the scaler mode) for the self-calibration of its surface detector array. From observations using the scaler mode at the Pierre Auger Observatory, modulation of galactic cosmic rays due to solar transient activity has been observed (e.g., Forbush decreases). Due to the high total count rate coming from the combined area of its detectors, the Pierre Auger Observatory (its detectors have a total area greater than $16\,000$\,m$^2$) detects a flux of secondary particles of the order of $\sim 10^8$\,counts per minute. Time variations of the cosmic ray flux related to the activity of the heliosphere can be determined with high accuracy. In this paper we briefly describe the scaler mode and analyze a Forbush decrease together with the interplanetary coronal mass ejection that originated it. The Auger scaler data are now publicly available.

Electron-ion Recombination of Fe XII forming Fe XI: Laboratory Measurements and Theoretical Calculations

astro-ph — Mon, 30 Apr 2012 00:46:38 +0000

We have measured electron-ion recombination for Fe XII forming Fe XI using a merged beams configuration at the heavy-ion storage ring TSR located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. The measured merged beams recombination rate coefficient (MBRRC) for collision energies from 0 to 1500 eV is presented. This work uses a new method for determining the absolute MBRRC based on a comparison of the ion beam decay rate with and without the electron beam on. For energies below 75 eV, the spectrum is dominated by dielectronic recombination (DR) resonances associated with 3s-3p and 3p-3d core excitations. At higher energies we observe contributions from 3-N’ and 2-N’ core excitations DR. We compare our experimental results to state-of-the-art multi-configuration Breit-Pauli (MCBP) calculations and find significant differences, both in resonance energies and strengths. We have extracted the DR contributions from the measured MBRRC data and transformed them into a plasma recombination rate coefficient (PRRC) for temperatures in the range of 10^3 to 10^7 K. We show that the previously recommended DR data for Fe XII significantly underestimate the PRRC at temperatures relevant for both photoionized plasmas (PPs) and collisionaly ionized plasmas (CPs). This is to be contrasted with our MCBP PRRC results which agree with the experiment to within 30% at PP temperatures and even better at CP temperatures. We find this agreement despite the disagreement shown by the detailed comparison between our MCBP and experimental MBRRC results. Lastly, we present a simple parameterized form of the experimentally derived PRRC for easy use in astrophysical modelling codes.

Light Yield in DarkSide-10: a Prototype Two-phase Liquid Argon TPC for Dark Matter Searches

astro-ph — Mon, 30 Apr 2012 00:46:25 +0000

As part of the DarkSide program of direct dark matter searches using liquid argon TPCs, a prototype detector with an active volume containing 10 kg of liquid argon, DarkSide-10, was built and operated underground in the Gran Sasso National Laboratory in Italy. A critically important parameter for such devices is the scintillation light yield, as photon statistics limits the rejection of electron-recoil backgrounds by pulse shape discrimination. We have measured the light yield of DarkSide-10 using the readily-identifiable full-absorption peaks from gamma ray sources combined with single-photoelectron calibrations using low-occupancy laser pulses. For gamma lines of energies in the range 122-1275 keV, we get consistent light yields averaging 8.887\pm0.003(stat)\pm0.444(sys) p.e./keV_ee. With additional purification, the light yield measured at 511 keV increased to 9.142\pm0.006(stat) p.e./keV_ee.

A Comparative Study of Local Galaxy Clusters: I. Derived X-ray Observables

astro-ph — Mon, 30 Apr 2012 00:38:58 +0000

We examine systematic differences in the derived X-ray properties of galaxy clusters as reported by three different groups: Vikhlinin et al. (2009a), Mantz et al. (2010b), and Planck Collaboration (2011b). The sample overlap between any two pairs of works ranges between 16 to 28 galaxy clusters in common. We find systematic differences in most reported properties, including the total cluster mass, M500. The most extreme case is an average 45% \pm 5% difference in cluster mass between the Planck Collaboration (2011b) and Mantz et al. (2010b), for clusters at z > 0.13 (averaged over 16 clusters). These mass differences induce differences in cluster observables defined within an R500 aperture. After accounting for aperture differences, we find very good agreement in gas mass estimates between the different groups. However, the soft-band X-ray luminosity, LX, core-excised spectroscopic temperature, TX, and gas thermal energy, YX = MgasTX display mean differences at the 5%-15% level. We also find that the low (z \leq 0.13) and high (z \geq 0.13) galaxy cluster samples in Planck Collaboration (2011b) appear to be systematically different: the YSZ/YX ratio for these two sub- samples is ln(YSZ/YX) = -0.06 \pm 0.04 and ln(YSZ/YX) = 0.08 \pm 0.04 for the low and high redshift sub-samples respectively.

A New MHD Code with Adaptive Mesh Refinement and Parallelization for Astrophysics

astro-ph — Fri, 27 Apr 2012 00:47:54 +0000

A new code, named MAP, is written in Fortran language for magnetohydrodynamics (MHD) calculation with the adaptive mesh refinement (AMR) and Message Passing Interface (MPI) parallelization. There are several optional numerical schemes for computing the MHD part, namely, modified Mac Cormack Scheme (MMC), Lax-Friedrichs scheme (LF) and weighted essentially non-oscillatory (WENO) scheme. All of them are second order, two-step, component-wise schemes for hyperbolic conservative equations. The total variation diminishing (TVD) limiters and approximate Riemann solvers are also equipped. A high resolution can be achieved by the hierarchical block-structured AMR mesh. We use the extended generalized Lagrange multiplier (EGLM) MHD equations to reduce the non-divergence free error produced by the scheme in the magnetic induction equation. The numerical algorithms for the non-ideal terms, e.g., the resistivity and the thermal conduction, are also equipped in the MAP code. The details of the AMR and MPI algorithms are described in the paper.

First Large Scale Production of Low Radioactivity Argon From Underground Sources

astro-ph — Fri, 27 Apr 2012 00:39:22 +0000

Accurate Group Delay Measurement for Radial Velocity Instruments Using the Dispersed Fixed Delay Interferometer Method [Replacement]

astro-ph — Fri, 27 Apr 2012 00:39:00 +0000

The dispersed fixed-delay Intereferometer (DFDI) method is attractive for its low cost, compact size, and multiobject capability in precision radial-velocity (RV) measurements. The phase shift of fringes of stellar absorption lines is measured and then converted to an RV shift via an important parameter, phase-to-velocity scale (PV scale), determined by the group delay (GD) of a fixed-delay interferometer. Two methods of GD measurement using a DFDI Doppler instrument are presented in this article: (1) GD measurement using white-light combs gen- erated by the fixed-delay interferometer and (2) GD calibration using an RV reference star. These two methods provide adequate precision of GD measurement and calibration, given the current RV precision achieved by a DFDI Doppler instrument. They can potentially be used to measure GD of an fixed-delay interferometer for submeter- precision Doppler measurement with a DFDI instrument. Advantages and limitations of each method are discussed in detail. The two methods can serve as standard procedures of PV-scale calibration for DFDI instruments and cross- checks for each other.

Four Fundamental Foreground Power Spectrum Shapes for 21 cm Cosmology Observations [Replacement]

astro-ph — Fri, 27 Apr 2012 00:34:16 +0000

Contamination from instrumental effects interacting with bright astrophysical sources is the primary impediment to measuring Epoch of Reionization and BAO 21 cm power spectra—an effect called mode-mixing. In this paper we identify four fundamental power spectrum shapes produced by mode-mixing that will affect all upcoming observations. We are able, for the first time, to explain the wedge-like structure seen in advanced simulations and to forecast the shape of an ‘EoR window’ that is mostly free of contamination. Understanding the origins of these contaminations also enables us to identify calibration and foreground subtraction errors below the imaging limit, providing a powerful new tool for precision observations.

Multi-physics simulations using a hierarchical interchangeable software interface

astro-ph — Thu, 26 Apr 2012 00:53:45 +0000

We introduce a general-purpose framework for interconnecting scientific simulation programs using a homogeneous, unified interface. Our framework is intrinsically parallel, and conveniently separates all component numerical modules in memory. This strict separation allows automatic unit conversion, distributed execution of modules on different cores within a cluster or grid, and orderly recovery from errors. The framework can be efficiently implemented and incurs an acceptable overhead. In practice, we measure the time spent in the framework to be less than 1% of the wall-clock time. Due to the unified structure of the interface, incorporating multiple modules addressing the same physics in different ways is relatively straightforward. Different modules may be advanced serially or in parallel. Despite initial concerns, we have encountered relatively few problems with this strict separation between modules, and the results of our simulations are consistent with earlier results using more traditional monolithic approaches. This framework provides a platform to combine existing simulation codes or develop new physical solver codes within a rich “ecosystem” of interchangeable modules.

Numerical Simulations of Optically Thick Accretion onto a Black Hole – I. Spherical Case

astro-ph — Thu, 26 Apr 2012 00:52:22 +0000

Career situation of female astronomers in Germany

astro-ph — Thu, 26 Apr 2012 00:46:12 +0000

We survey the job situation of women in astronomy in Germany and of German women abroad and review indicators for their career development. Our sample includes women astronomers from all academic levels from doctoral students to professors, as well as female astronomers who have left the field. We find that networking and human support are among the most important factors for success. Experience shows that students should carefully choose their supervisor and collect practical knowledge abroad. We reflect the private situation of female German astronomers and find that prejudices are abundant, and are perceived as discriminating.We identify reasons why women are more likely than men to quit astronomy after they obtain their PhD degree. We give recommendations to young students on what to pay attention to in order to be on the successful path in astronomy.

Recognizing magnetic structures by present and future radio telescopes with RM Synthesis

astro-ph — Thu, 26 Apr 2012 00:44:14 +0000

Detecting transient gravitational waves in non-Gaussian noise with partially redundant analysis methods [Replacement]

astro-ph — Thu, 26 Apr 2012 00:37:06 +0000

There is a broad class of astrophysical sources that produce detectable, transient, gravitational waves. Some searches for transient gravitational waves are tailored to known features of these sources. Other searches make few assumptions about the sources. Typically events are observable with multiple search techniques. This work describes how to combine the results of searches that are not independent, treating each search as a classifier for a given event. This will be shown to improve the overall sensitivity to gravitational-wave events while directly addressing the problem of consistent interpretation of multiple trials.

The BSSN equations in spherical coordinates without regularization: vacuum and non-vacuum spherically symmetric spacetimes [Cross-Listing]

astro-ph — Thu, 26 Apr 2012 00:31:07 +0000

Brown has recently introduced a covariant formulation of the BSSN equations which is well suited for curvilinear coordinate systems. This is particularly desirable as many astrophysical phenomena are symmetric with respect to the rotation axis or are such that curvilinear coordinates adapt better to their geometry. However, the singularities associated with such coordinate systems are known to lead to numerical instabilities unless special care is taken (e.g., regularization at the origin). Cordero-Carrion will present a rigorous derivation of partially implicit Runge-Kutta methods in forthcoming papers, with the aim of treating numerically the stiff source terms in wave-like equations that may appear as a result of the choice of the coordinate system. We have developed a numerical code solving the BSSN equations in spherical symmetry and the general relativistic hydrodynamic equations written in flux-conservative form. A key feature of the code is that it uses a second-order partially implicit Runge-Kutta method to integrate the evolution equations. We perform and discuss a number of tests to assess the accuracy and expected convergence of the code, namely a pure gauge wave, the evolution of a single black hole, the evolution of a spherical relativistic star in equilibrium, and the gravitational collapse of a spherical relativistic star leading to the formation of a black hole. We obtain stable evolutions of regular spacetimes without the need for any regularization algorithm at the origin.

High-resolution optical imaging of the core of the globular cluster M15 with FastCam

astro-ph — Wed, 25 Apr 2012 00:47:59 +0000

We present high-resolution I-band imaging of the core of the globular cluster M15 obtained at the 2.5 m Nordic Optical Telescope with FastCam, a low readout noise L3CCD based instrument. Short exposure times (30 ms) were used to record 200000 images (512 x 512 pixels each) over a period of 2 hours 43 min. The lucky imaging technique was then applied to generate a final image of the cluster centre with FWHM ~ 0″.1 and 13″ x 13″ FoV. We obtained a catalogue of objects in this region with a limiting magnitude of I=19.5. I-band photometry and astrometry are reported for 1181 stars. This is the deepest I-band observation of the M15 core at this spatial resolution. Simulations show that crowding is limiting the completeness of the catalogue. At shorter wavelengths, a similar number of objects has been reported using HST/WFPC observations of the same field. The cross-match with the available HST catalogues allowed us to produce colour-magnitude diagrams where we identify new Blue Straggler star candidates and previously known stars of this class.

Construction of a Calibrated Probabilistic Classification Catalog: Application to 50k Variable Sources in the All-Sky Automated Survey [Replacement]

astro-ph — Wed, 25 Apr 2012 00:32:46 +0000

With growing data volumes from synoptic surveys, astronomers must become more abstracted from the discovery and introspection processes. Given the scarcity of follow-up resources, there is a particularly sharp onus on the frameworks that replace these human roles to provide accurate and well-calibrated probabilistic classification catalogs. Such catalogs inform the subsequent follow-up, allowing consumers to optimize the selection of specific sources for further study and permitting rigorous treatment of purities and efficiencies for population studies. Here, we describe a process to produce a probabilistic classification catalog of variability with machine learning from a multi-epoch photometric survey. In addition to producing accurate classifications, we show how to estimate calibrated class probabilities, and motivate the importance of probability calibration. We also introduce a methodology for feature-based anomaly detection, which allows discovery of objects in the survey that do not fit within the predefined class taxonomy. Finally, we apply these methods to sources observed by the All Sky Automated Survey (ASAS), and unveil the Machine-learned ASAS Classification Catalog (MACC), which is a 28-class probabilistic classification catalog of 50,124 ASAS sources. We estimate that MACC achieves a sub-20% classification error rate, and demonstrate that the class posterior probabilities are reasonably calibrated. MACC classifications compare favorably to the classifications of several previous domain-specific ASAS papers and to the ASAS Catalog of Variable Stars, which had classified only 24% of those sources into one of 12 science classes. The MACC is publicly available at http://www.bigmacc.info.

A Per-Baseline, Delay-Spectrum Technique for Accessing the 21cm Cosmic Reionization Signature

astro-ph — Tue, 24 Apr 2012 00:48:52 +0000

A critical challenge in measuring the power spectrum of 21cm emission from cosmic reionization is compensating for the frequency dependence of an interferometer’s sampling pattern, which can cause smooth-spectrum foregrounds to appear unsmooth and degrade the separation between foregrounds and the target signal. In this paper, we present an approach to foreground removal that explicitly accounts for this frequency dependence. We apply the delay transformation introduced in Parsons & Backer (2009) to each baseline of an interferometer to concentrate smooth-spectrum foregrounds within the bounds of the maximum geometric delays physically realizable on that baseline. By focusing on delay-modes that correspond to image-domain regions beyond the horizon, we show that it is possible to avoid the bulk of smooth-spectrum foregrounds. We show that delay-modes that are uncorrupted by foregrounds also represent samples of the three-dimensional power spectrum, and can be used to constrain cosmic reionization. Because it uses only spectral smoothness to differentiate foregrounds from the targeted 21cm signature, this per-baseline analysis approach relies on spectrally- and spatially-smooth instrumental responses for foreground removal. For sufficient levels of instrumental smoothness relative to the brightness of interfering foregrounds, this technique substantially reduces the level of calibration previously thought necessary to detect 21cm reionization. As a result, this approach places fewer constraints on antenna configuration within an array, and in particular, facilitates the adoption of configurations that are optimized for power-spectrum sensitivity. Under these assumptions, we demonstrate the potential for the PAPER array to detect 21cm reionization at an amplitude of 10 mK^2 near k~0.2h Mpc^-1 with 128 dipoles in 7 months of observing.

Paraiso : An Automated Tuning Framework for Explicit Solvers of Partial Differential Equations

astro-ph — Tue, 24 Apr 2012 00:48:03 +0000

The LWA1 Radio Telescope

astro-ph — Tue, 24 Apr 2012 00:46:34 +0000

Calibration systems of the ANTARES neutrino telescope

astro-ph — Tue, 24 Apr 2012 00:39:12 +0000

The ANTARES detector is the largest deep sea underwater neutrino telescope in operation. The apparatus comprises a matrix of 885 photomultiplier tubes (PMTs) which detect the Cherenkov light emitted by the charged leptons produced in the charged current interactions of high energy neutrinos with the matter inside or near the detector. Reconstruction of the muon track and energy can be achieved using the time, position and charge information of the hits arriving to the PMTs. A good calibration of the detector is necessary in order to ensure its optimal performance. This contribution reviews the different calibration systems and methods developed by the ANTARES Collaboration.

The JEM-EUSO Mission: Status and Prospects in 2011

astro-ph — Tue, 24 Apr 2012 00:37:34 +0000

Contributions of the JEM-EUSO Collaboration to the 32nd International Cosmic Ray Conference, Beijing, August, 2011.

Field Programmable Gate Array based Front-End Data Acquisition Module for the COSMICi Astroparticle Telescope System

astro-ph — Tue, 24 Apr 2012 00:36:45 +0000

We describe an FPGA based Front-End Data Acquisition Module (FEDAM) for implementing Time-over-Threshold (ToT) Time-to-Digital conversion (TDC) of pulses obtained from the COSMICi astroparticle telescope detector system photomultiplier tubes. The telescope system consists of a minimum of three scintillation detectors configured to detect particle airshowers likely initiated by Ultra High Energy Cosmic Rays (UHECR). The relative time delay of detection events between the detectors is used to estimate the angle of incidence of the shower. The FEDAM provides time-over-threshold measurements with a resolution of 2 ns. This allows determination of shower direction to an error of 0.035 (cos {\theta})-1 radians where {\theta} is the angle between the baseline axis through a pair of detectors and the plane representing the shower front.

Means of confusion: how pixel noise affects shear estimates for weak gravitational lensing

astro-ph — Tue, 24 Apr 2012 00:36:02 +0000

Weak-lensing shear estimates show a troublesome dependence on the apparent brightness of the galaxies used to measure the ellipticity: In several studies, the amplitude of the inferred shear falls sharply with decreasing source significance. This dependence limits the overall ability of upcoming large weak-lensing surveys to constrain cosmological parameters. We seek to provide a concise overview of the impact of pixel noise on weak-lensing measurements, covering the entire path from noisy images to shear estimates. We show that there are at least three distinct layers, where pixel noise not only obscures but biases the outcome of the measurements: 1) the propagation of pixel noise to the non-linear observable ellipticity; 2) the response of the shape-measurement methods to limited amount of information extractable from noisy images; and 3) the reaction of shear estimation statistics to the presence of noise and outliers in the measured ellipticities. We identify and discuss several fundamental problems and show that each of them is able to introduce biases in the range of a few tenths to a few percent for galaxies with typical significance levels. Furthermore, all of these biases do not only depend on the brightness of galaxies but also on their ellipticity, with more elliptical galaxies often being harder to measure correctly. We also discuss existing possibilities to mitigate and novel ideas to avoid the biases induced by pixel noise. We present a new shear estimator that shows a more robust performance for noisy ellipticity samples. Finally, we release the open-source python code to predict and efficiently sample from the noisy ellipticity distribution and the shear estimators used in this work at https://github.com/pmelchior/epsnoise.

The WFIRST Galaxy Survey Exposure Time Calculator

astro-ph — Tue, 24 Apr 2012 00:35:44 +0000

This document describes the exposure time calculator for the Wide-Field Infrared Survey Telescope (WFIRST) high-latitude survey. The calculator works in both imaging and spectroscopic modes. In addition to the standard ETC functions (e.g. background and S/N determination), the calculator integrates over the galaxy population and forecasts the density and redshift distribution of galaxy shapes usable for weak lensing (in imaging mode) and the detected emission lines (in spectroscopic mode). The source code is made available for public use.

Weak gravitational lensing with DEIMOS [Replacement]

astro-ph — Tue, 24 Apr 2012 00:35:18 +0000

We introduce a novel method for weak-lensing measurements, which is based on a mathematically exact deconvolution of the moments of the apparent brightness distribution of galaxies from the telescope’s PSF. No assumptions on the shape of the galaxy or the PSF are made. The (de)convolution equations are exact for unweighted moments only, while in practice a compact weight function needs to be applied to the noisy images to ensure that the moment measurement yields significant results. We employ a Gaussian weight function, whose centroid and ellipticity are iteratively adjusted to match the corresponding quantities of the source. The change of the moments caused by the application of the weight function can then be corrected by considering higher-order weighted moments of the same source. Because of the form of the deconvolution equations, even an incomplete weighting correction leads to an excellent shear estimation if galaxies and PSF are measured with a weight function of identical size. We demonstrate the accuracy and capabilities of this new method in the context of weak gravitational lensing measurements with a set of specialized tests and show its competitive performance on the GREAT08 challenge data. A complete C++ implementation of the method can be requested from the authors.

Evolution and Hydrodynamics of the Very-Broad X-ray Line Emission in SN 1987A [Replacement]

astro-ph — Tue, 24 Apr 2012 00:33:21 +0000

Observations of SN 1987A by the Chandra High Energy Transmission Grating (HETG) in 1999 and the XMM-Newton Reflection Grating Spectrometer (RGS) in 2003 show very broad (v-b) lines with a full-width at half-maximum (FWHM) of order 10^4 kms; at these times the blast wave was primarily interacting with the HII region around the progenitor. Since then, the X-ray emission has been increasingly dominated by narrower components as the blast wave encounters dense equatorial ring (ER) material. Even so, continuing v-b emission is seen in the grating spectra suggesting that interaction with HII region material is on-going. Based on the deep HETG 2007 and 2011 data sets, and confirmed by RGS and other HETG observations, the v-b component has a width of 9300 +/-2000 kms FWHM and contributes of order 20% of the current 0.5–2 keV flux. Guided by this result, SN 1987A’s X-ray spectra are modeled as the weighted sum of the non-equilibrium-ionization (NEI) emission from two simple 1D hydrodynamic simulations, this “2×1D” model reproduces the observed radii, light curves, and spectra with a minimum of free parameters. The interaction with the HII region (rho_init \sim 130 amu/cc, +/- 15 degrees opening angle) produces the very-broad emission lines and most of the 3-10 keV flux. Our ER hydrodynamics, admittedly a crude approximation to the multi-D reality, gives ER densities of order 10^4 amu/cc, requires dense clumps (x5.5 density enhancement in \sim 30% of the volume), and it predicts that the 0.5-2 keV flux will drop at a rate of \sim 17% per year once no new dense ER material is being shocked.

Improved visual detection of moving objects in astronomical images using color intensity projections with hue cycling [Replacement]

astro-ph — Tue, 24 Apr 2012 00:33:02 +0000

Measuring the solar atmosphere

astro-ph — Mon, 23 Apr 2012 00:48:43 +0000

The new CRISP filter at the Swedish 1-m Solar Telescope provides opportunities for observing the solar atmosphere with unprecedented spatial resolution and cadence. In order to benefit from the high quality of observational data from this instrument, we have developed methods for calibrating and restoring polarized Stokes images, obtained at optical and near infrared wavelengths, taking into account field-of-view variations of the filter properties. In order to facilitate velocity measurements, a time series from a 3D hydrodynamical granulation simulation is used to compute quiet Sun spectral line profiles at different heliocentric angles. The synthetic line profiles, with their convective blueshifts, can be used as absolute references for line-of-sight velocities. Observations of the Ca II 8542 {\AA} line are used to study magnetic fields in chromospheric fibrils. The line wings show the granulation pattern at mid-photospheric heights whereas the overlying chromosphere is seen in the core of the line. Using full Stokes data, we have attempted to observationally verify the alignment of chromospheric fibrils with the magnetic field. Our results suggest that in most cases fibrils are aligned along the magnetic field direction, but we also find examples where this is not the case. Detailed interpretation of Stokes data from spectral lines formed in the chromospheric data can be made using non-LTE inversion codes. For the first time, we use a realistic 3D MHD chromospheric simulation of the quiet Sun to assess how well NLTE inversions recover physical quantities from spectropolarimetric observations of Ca II 8542 {\AA}. We demonstrate that inversions provide realistic estimates of depth-averaged quantities in the chromosphere, although high spectral resolution and high sensitivity are needed to measure quiet Sun chromospheric magnetic fields.

A multi-scale, multi-wavelength source extraction method: getsources

astro-ph — Mon, 23 Apr 2012 00:41:37 +0000

We present a multi-scale, multi-wavelength source extraction algorithm called getsources. Although it has been designed primarily for use in the far-infrared surveys of Galactic star-forming regions with Herschel, the method can be applied to many other astronomical images. Instead of the traditional approach of extracting sources in the observed images, the new method analyzes fine spatial decompositions of original images across a wide range of scales and across all wavebands. It cleans those single-scale images of noise and background, and constructs wavelength-independent single-scale detection images that preserve information in both spatial and wavelength dimensions. Sources are detected in the combined detection images by following the evolution of their segmentation masks across all spatial scales. Measurements of the source properties are done in the original background-subtracted images at each wavelength; the background is estimated by interpolation under the source footprints and overlapping sources are deblended in an iterative procedure. In addition to the main catalog of sources, various catalogs and images are produced that aid scientific exploitation of the extraction results. We illustrate the performance of getsources on Herschel images by extracting sources in sub-fields of the Aquila and Rosette star-forming regions. The source extraction code and validation images with a reference extraction catalog are freely available.

Optimizing optical follow-up of gravitational-wave candidates

astro-ph — Mon, 23 Apr 2012 00:41:26 +0000

Observations with interferometric gravitational-wave detectors result in probability sky maps that are multimodal and spread over 10-100 deg^2. We present a scheme for maximizing the probability of imaging optical counterparts to gravitational-wave transients given limited observing resources. Our framework is capable of coordinating many telescopes with different fields of view and limiting magnitudes. We present a case study comparing three different planning algorithms. We find that, with the network of telescopes that was used in the most recent joint LIGO-Virgo science run, a relatively straightforward coordinated approach doubles the detection efficiency relative to each telescope observing independently.

VLBI for Gravity Probe B. I. Overview

astro-ph — Mon, 23 Apr 2012 00:38:05 +0000

We describe the NASA/Stanford gyroscope relativity mission, Gravity Probe B (GP-B), and provide an overview of the following series of six astrometric and astrophysical papers that report on our radio observations and analyses made in support of this mission. The main goal of this 8.5 year program of differential VLBI astrometry was to determine the proper motion of the guide star of the GP-B mission, the RS CVn binary IM Pegasi (IM Peg; HR 8703). This proper motion is determined with respect to compact, extragalactic reference sources. The results are: -20.833 +- 0.090 mas/yr and -27.267 +- 0.095 mas/yr for, respectively, the right ascension and declination, in local Cartesian coordinates, of IM Peg’s proper motion, and 10.370 +- 0.074 mas (i.e., 96.43 +- 0.69 pc) for its parallax (and distance). Each quoted uncertainty is meant to represent an ~70% confidence interval that includes the estimated contribution from systematic error. These results are accurate enough not to discernibly degrade the GP-B estimates of its gyroscopes’ relativistic precessions: the frame-dragging and geodetic effects.

VLBI for Gravity Probe B. III. A Limit on the Proper Motion of the "Core" of the Quasar 3C 454.3

astro-ph — Mon, 23 Apr 2012 00:37:29 +0000

We made VLBI observations at 8.4 GHz between 1997 and 2005 to estimate the coordinates of the “core” component of the superluminal quasar, 3C 454.3, the ultimate reference point in the distant universe for the NASA/Stanford Gyroscope Relativity Mission, Gravity Probe B. These coordinates are determined relative to those of the brightness peaks of two other compact extragalactic sources, B2250+194 and B2252+172, nearby on the sky, and within a celestial reference frame (CRF), defined by a large suite of compact extragalactic radio sources, and nearly identical to the International Celestial Reference Frame 2 (ICRF2). We find that B2250+194 and B2252+172 are stationary relative to each other, and also in the CRF, to within 1-sigma upper limits of 15 and 30 micro-arcsec/yr in RA and decl., respectively. The core of 3C 454.3 appears to jitter in its position along the jet direction over ~0.2 mas, likely due to activity close to the putative supermassive black hole nearby, but on average is stationary in the CRF within 1-sigma upper limits on its proper motion of 39 micro-arcsec/yr (1.0c) and 30 micro-arcsec/yr (0.8c) in RA and decl., respectively, for the period 2002 – 2005. Our corresponding limit over the longer interval, 1998 – 2005, of more importance to GP-B, is 46 and 56 micro-arcsec/yr in RA and decl., respectively. Some of 3C 454.3’s jet components show significantly superluminal motion with speeds of up to ~200 micro-arcsec/yr or 5c in the CRF. The core of 3C 454.3 thus provides for Gravity Probe B a sufficiently stable reference in the distant universe.

VLBI for Gravity Probe B. V. Proper Motion and Parallax of the Guide Star, IM Pegasi

astro-ph — Mon, 23 Apr 2012 00:37:18 +0000

We present the principal astrometric results of the very-long-baseline interferometry (VLBI) program undertaken in support of the Gravity Probe B (GP-B) relativity mission. VLBI observations of the GP-B guide star, the RS CVn binary IM Pegasi (HR 8703), yielded positions at 35 epochs between 1997 and 2005. We discuss the statistical assumptions behind these results and our methods for estimating the systematic errors. We find the proper motion of IM Peg in an extragalactic reference frame closely related to the International Celestial Reference Frame 2 (ICRF2) to be -20.83 +- 0.03 +- 0.09 mas/yr in right ascension and -27.27 +- 0.03 +- 0.09 mas/yr in declination. For each component the first uncertainty is the statistical standard error and the second is the total standard error (SE) including plausible systematic errors. We also obtain a parallax of 10.37 +- 0.07 mas (distance: 96.4 +- 0.7 pc), for which there is no evidence of any significant contribution of systematic error. Our parameter estimates for the ~25-day-period orbital motion of the stellar radio emission have SEs corresponding to ~0.10 mas on the sky in each coordinate. The total SE of our estimate of IM Peg’s proper motion is ~30% smaller than the accuracy goal set by the GP-B project before launch: 0.14 mas/yr for each coordinate of IM Peg’s proper motion. Our results ensure that the uncertainty in IM Peg’s proper motion makes only a very small contribution to the uncertainty of the GP-B relativity tests.

VLBI for Gravity Probe B. IV. A New Astrometric Analysis Technique and a Comparison with Results from Other Techniques

astro-ph — Mon, 23 Apr 2012 00:37:07 +0000

When VLBI observations are used to determine the position or motion of a radio source relative to reference sources nearby on the sky, the astrometric information is usually obtained via: (i) phase-referenced maps; or (ii) parametric model fits to measured fringe phases or multiband delays. In this paper we describe a “merged” analysis technique which combines some of the most important advantages of these other two approaches. In particular, our merged technique combines the superior model-correction capabilities of parametric model fits with the ability of phase-referenced maps to yield astrometric measurements of sources that are too weak to be used in parametric model fits. We compare the results from this merged technique with the results from phase-referenced maps and from parametric model fits in the analysis of astrometric VLBI observations of the radio-bright star IM Pegasi (HR 8703) and the radio source B2252+172 nearby on the sky. In these studies we use central-core components of radio sources 3C 454.3 and B2250+194 as our positional references. We obtain astrometric results for IM Peg with our merged technique even when the source is too weak to be used in parametric model fits, and we find that our merged technique yields superior astrometric results to the phase-referenced mapping technique. We used our merged technique to estimate the proper motion and other astrometric parameters of IM Peg in support of the NASA/Stanford Gravity Probe B mission.

VLBI for Gravity Probe B. II. Monitoring of the Structure of the Reference Sources 3C 454.3, B2250+194, and B2252+172

astro-ph — Mon, 23 Apr 2012 00:36:54 +0000

We used 8.4 GHz VLBI images obtained at up to 35 epochs between 1997 and 2005 to examine the radio structures of the main reference source, 3C 454.3, and two secondary reference sources, B2250+194 and B2252+172, for the guide star for the NASA/Stanford relativity mission Gravity Probe B (GP-B). For one epoch in 2004 May, we also obtained images at 5.0 and 15.4 GHz. The 35 8.4 GHz images for quasar 3C 454.3 confirm a complex, evolving, core-jet structure. We identified at each epoch a component, C1, near the easternmost edge of the core region. Simulations of the core region showed that C1 is located, on average, 0.18 +- 0.06 mas west of the unresolved “core” identified in 43 GHz images. We also identified in 3C 454.3 at 8.4 GHz several additional components which moved away from C1 with proper motions ranging in magnitude between 0.9c and 5c. The detailed motions of the components exhibit two distinct bends in the jet axis located ~3 and ~5.5 mas west of C1. The spectra between 5.0 and 15.4 GHz for the “moving” components are steeper than that for C1. The 8.4 GHz images of B2250+194 and B2252+172, in contrast to those of 3C 454.3, reveal compact structures. The spectrum between 5.0 and 15.4 GHz for B2250+194 is inverted while that for B2252+172 is flat. Based on its position near the easternmost edge of the 8.4 GHz radio structure, close spatial association with the 43 GHz core, and relatively flat spectrum, we believe 3C 454.3 component C1 to be the best choice for the ultimate reference point for the GP-B guide star. The compact structures and inverted to flat spectra of B2250+194 and B2252+172 make these objects valuable secondary reference sources

VLBI for Gravity Probe B. VI. The Orbit of IM Pegasi and the Location of the Source of Radio Emission

astro-ph — Mon, 23 Apr 2012 00:36:37 +0000

We present a physical interpretation for the locations of the sources of radio emission in IM Pegasi (IM Peg, HR 8703), the guide star for the NASA/Stanford relativity mission Gravity Probe B. This emission is seen in each of our 35 epochs of 8.4-GHz VLBI observations taken from 1997 to 2005. We found that the mean position of the radio emission is at or near the projected center of the primary to within about 27% of its radius, identifying this active star as the radio emitter. The positions of the radio brightness peaks are scattered across the disk of the primary and slightly beyond, preferentially along an axis with position angle, p.a. = (-38 +- 8) deg, which is closely aligned with the sky projections of the orbit normal (p.a. = -49.5 +- 8.6 deg) and the expected spin axis of the primary. Comparison with simulations suggests that brightness peaks are 3.6 (+0.4,-0.7) times more likely to occur (per unit surface area) near the pole regions of the primary (|latitude| >= 70 deg) than near the equator (|latitude| <= 20 deg), and to also occur close to the surface with ~2/3 of them at altitudes not higher than 25% of the radius of the primary.

VLBI for Gravity Probe B. VII. The Evolution of the Radio Structure of IM Pegasi

astro-ph — Mon, 23 Apr 2012 00:36:26 +0000

We present measurements of the total radio flux density as well as very-long-baseline interferometry (VLBI) images of the star, IM Pegasi, which was used as the guide star for the NASA/Stanford relativity mission Gravity Probe B. We obtained flux densities and images from 35 sessions of observations at 8.4 GHz (wavelength = 3.6 cm) between 1997 January and 2005 July. The observations were accurately phase-referenced to several extragalactic reference sources, and we present the images in a star-centered frame, aligned by the position of the star as derived from our fits to its orbital motion, parallax, and proper motion. Both the flux density and the morphology of IM Peg are variable. For most sessions, the emission region has a single-peaked structure, but 25% of the time, we observed a two-peaked (and on one occasion perhaps a three-peaked) structure. On average, the emission region is elongated by 1.4 +- 0.4 mas (FWHM), with the average direction of elongation being close to that of the sky projection of the orbit normal. The average length of the emission region is approximately equal to the diameter of the primary star. No significant correlation with the orbital phase is found for either the flux density or the direction of elongation, and no preference for any particular longitude on the star is shown by the emission region.

Bifurcation in the Presence of Focusing

astro-ph — Mon, 23 Apr 2012 00:35:29 +0000

A number of physical processes show some form of bifurcation or splintering around a given point. Recently, it has been noted that cavity searches for interactions between photons and exotic fields may also show a bifurcation[1]. This paper describes the simulation of bifurcation of an optical beam in the presence of periodic focusing. The work is applicable to searches for exotic particles that employ cavities to extend the optical path of a laser beam through an interacting field. Significantly, simulations of cavity bifurcation reveal both a redistribution of the beam’s energy density and a shifting of the beam’s profile.

Prototyping scalable digital signal processing systems for radio astronomy using dataflow models

astro-ph — Mon, 23 Apr 2012 00:35:18 +0000

There is a growing trend toward using high-level tools for design and implementation of radio astronomy digital signal processing (DSP) systems. Such tools, for example, those from the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER), are usually platform-specific, and lack high-level, platform-independent, portable, scalable application specifications. This limits the designer’s ability to experiment with designs at a high-level of abstraction and early in the development cycle. We address some of these issues using a model-based design approach employing dataflow models. We demonstrate this approach by applying it to the design of a tunable digital downconverter (TDD) used for narrow-bandwidth spectroscopy. Our design is targeted toward an FPGA platform, called the Interconnect Break-out Board (IBOB), that is available from the CASPER. We use the term TDD to refer to a digital downconverter for which the decmation factor and center frequency can be reconfigured without the need for regenerating the hardware code. Such a design is currently not available in the CASPER DSP library. The work presented in this paper focuses on two aspects. Firstly, we introduce and demonstrate a dataflow-based design approach using the dataflow interchange format (DIF) tool for high-level application specification, and we integrate this approach with the CASPER tool flow. Secondly, we explore the trade-off between the flexibility of TDD designs and the low hardware cost of fixed-configuration digital downconverter (FDD) designs that use the available CASPER DSP library. We further explore this trade-off in the context of a two-stage downconversion scheme employing a combination of TDD or FDD designs.

Intensity and polarization of the atmospheric emission at millimetric wavelengths at Dome Concordia [Replacement]

astro-ph — Mon, 23 Apr 2012 00:32:08 +0000

Atmospheric emission is a dominant source of disturbance in ground-based astronomy at mm wavelengths. The Antarctic plateau is recognized to be an ideal site for mm and sub-mm observations, and the French/Italian base of Dome C is among the best sites on Earth for these observations. In this paper we present measurements, performed using the BRAIN-pathfinder experiment, at Dome C of the atmospheric emission in intensity and polarization at 150GHz, one of the best observational frequencies for CMB observations when considering cosmic signal intensity, atmospheric transmission, detectors sensitivity, and foreground removal. Careful characterization of the air-mass synchronous emission has been performed, acquiring more that 380 elevation scans (i.e. “skydip”) during the third BRAIN-pathfinder summer campaign in December 2009/January 2010. The extremely high transparency of the Antarctic atmosphere over Dome Concordia is proven by the very low measured optical depth: =0.050 \pm 0.003 \pm 0.011 where the first error is statistical and the second is systematic error. Mid term stability, over the summer campaign, of the atmosphere emission has also been studied. Adapting the radiative transfer atmosphere emission model “am” to the particular conditions found at Dome C, we also infer the level of the PWV content of the atmosphere, notoriously the main source of disturbance in millimetric astronomy (=0.77 +/- 0.06 + 0.15 – 0.12 mm). Upper limits on the air-mass correlated polarized signal are also placed for the first time. The degree of circular polarization of atmospheric emission is found to be lower than 0.2% (95%CL), while the degree of linear polarization is found to be lower than 0.1% (95%CL). These limits include signal-correlated instrumental spurious polarization.

Construction of a Calibrated Probabilistic Classification Catalog: Application to 50k Variable Sources in the All-Sky Automated Survey

astro-ph — Fri, 20 Apr 2012 01:04:17 +0000

With growing data volumes from synoptic surveys, astronomers must become more abstracted from the discovery and introspection processes. Given the scarcity of follow-up resources, there is a particularly sharp onus on the frameworks that replace these human roles to provide accurate and well-calibrated probabilistic classification catalogs. Such catalogs inform the subsequent follow-up, allowing consumers to optimize the selection of specific sources for further study and permitting rigorous treatment of purities and efficiencies for population studies. Here, we describe a process to produce a probabilistic classification catalog of variability with machine learning from a multi-epoch photometric survey. In addition to producing accurate classifications, we show how to estimate calibrated class probabilities, and motivate the importance of probability calibration. We also introduce a methodology for feature-based anomaly detection, which allows discovery of objects in the survey that do not fit within the predefined class taxonomy. Finally, we apply these methods to sources observed by the All Sky Automated Survey (ASAS), and unveil the Machine-learned ASAS Classification Catalog (MACC), which is a 28-class probabilistic classification catalog of 50,124 ASAS sources. We estimate that MACC achieves a sub-20% classification error rate, and demonstrate that the class posterior probabilities are reasonably calibrated. MACC classifications compare favorably to the classifications of several previous domain-specific ASAS papers and to the ASAS Catalog of Variable Stars, which had classified only 24% of those sources into one of 11 science classes. The MACC is publicly available at http://www.bigmacc.info.

Discovery of Bright Galactic R Coronae Borealis and DY Persei Variables: Rare Gems Mined from ASAS

astro-ph — Fri, 20 Apr 2012 01:03:39 +0000

We present the results of a machine-learning (ML) based search for new R Coronae Borealis (RCB) stars and DY Persei-like stars (DYPers) in the Galaxy using cataloged light curves obtained by the All-Sky Automated Survey (ASAS). RCB stars – a rare class of hydrogen-deficient carbon-rich supergiants – are of great interest owing to the insights they can provide on the late stages of stellar evolution. DYPers are possibly the low-temperature, low-luminosity analogs to the RCB phenomenon, though additional examples are needed to fully establish this connection. While RCB stars and DYPers are traditionally identified by epochs of extreme dimming that occur without regularity, the ML search framework more fully captures the richness and diversity of their photometric behavior. We demonstrate that our ML method recovers ASAS candidates that would have been missed by traditional search methods employing hard cuts on amplitude and periodicity. Our search yields 13 candidates that we consider likely RCB stars/DYPers: new and archival spectroscopic observations confirm that four of these candidates are RCB stars and four are DYPers. Our discovery of four new DYPers increases the number of known Galactic DYPers from two to six; noteworthy is that one of the new DYPers has a measured parallax and is m ~ 7 mag, making it the brightest known DYPer to date. Future observations of these new DYPers should prove instrumental in establishing the RCB connection. We consider these results, derived from a machine-learned probabilistic classification catalog, as an important proof-of-concept for the efficient discovery of rare sources with time-domain surveys.

Optimal strategies for continuous gravitational wave detection in pulsar timing arrays

astro-ph — Fri, 20 Apr 2012 00:55:22 +0000

Supermassive black hole binaries (SMBHBs) are expected to emit continuous gravitational waves in the pulsar timing array (PTA) frequency band ($10^{-9}$–$10^{-7}$ Hz). The development of data analysis techniques aimed at efficient detection and characterization of these signals is critical to the gravitational wave detection effort. In this paper we leverage methods developed for LIGO continuous wave gravitational searches, and explore the use of the $\mathcal{F}$-statistic for such searches in pulsar timing data. Babak & Sesana 2012 have already used this approach in the context of PTAs to show that one can resolve multiple SMBHB sources in the sky. Our work improves on several aspects of prior continuous wave search methods developed for PTA data analysis. The algorithm is implemented fully in the time domain, which naturally deals with the irregular sampling typical of PTA data and avoids spectral leakage problems associated with frequency domain methods. We take into account the fitting of the timing model, and have generalized our approach to deal with both correlated and uncorrelated colored noise sources. We also develop an incoherent detection statistic that maximizes over all pulsar dependent contributions to the likelihood. To test the effectiveness and sensitivity of our detection statistics, we perform a number of monte-carlo simulations. We produce sensitivity curves for PTAs of various configurations, and outline an implementation of a fully functional data analysis pipeline. Finally, we present a derivation of the likelihood maximized over the gravitational wave phases at the pulsar locations, which results in a vast reduction of the search parameter space.

The optimal schedule for pulsar timing array observations

astro-ph — Fri, 20 Apr 2012 00:48:39 +0000

In order to maximize the sensitivity of pulsar timing arrays to a stochastic gravitational wave background, we present computational techniques to optimize observing schedules. The techniques are applicable to both single and multi-telescope experiments. The observing schedule is optimized for each telescope by adjusting the observing time allocated to each pulsar while keeping the total amount of observing time constant. The optimized schedule depends on the timing noise characteristics of each individual pulsar as well as the performance of instrumentation. Several examples are given to illustrate the effects of different types of noise. A method to select the most suitable pulsars to be included in a pulsar timing array project is also presented.

Imaging the heart of astrophysical objects with optical long-baseline interferometry

astro-ph — Fri, 20 Apr 2012 00:45:31 +0000

The number of publications of aperture-synthesis images based on optical long-baseline interferometry measurements has recently increased due to easier access to visible and infrared interferometers. The interferometry technique has now reached a technical maturity level that opens new avenues for numerous astrophysical topics requiring milli-arcsecond model-independent imaging. In writing this paper our motivation was twofold: 1) review and publicize emblematic excerpts of the impressive corpus accumulated in the field of optical interferometry image reconstruction; 2) discuss future prospects for this technique by selecting four representative astrophysical science cases in order to review the potential benefits of using optical long baseline interferometers. For this second goal we have simulated interferometric data from those selected astrophysical environments and used state-of-the-art codes to provide the reconstructed images that are reachable with current or soon-to-be facilities. The image reconstruction process was “blind” in the sense that reconstructors had no knowledge of the input brightness distributions. We discuss the impact of optical interferometry in those four astrophysical fields. We show that image reconstruction software successfully provides accurate morphological information on a variety of astrophysical topics and review the current strengths and weaknesses of such reconstructions. We investigate how to improve image reconstruction and the quality of the image possibly by upgrading the current facilities. We finally argue that optical interferometers and their corresponding instrumentation, existing or to come, with 6 to 10 telescopes, should be well suited to provide images of complex sceneries.

The HARPS-TERRA project I. Description of the algorithms, performance and new measurements on a few remarkable stars observed by HARPS [Replacement]

astro-ph — Fri, 20 Apr 2012 00:35:19 +0000

Doppler spectroscopy has uncovered or confirmed all the known planets orbiting nearby stars. Two main techniques are used to obtain precision Doppler measurements at optical wavelengths. The first approach is the gas cell method, which consists on the least-squares matching of the spectrum of Iodine imprinted on the spectrum of the star. The second method relies on the construction of a stabilized spectrograph externally calibrated in wavelength. The most precise stabilized spectrometer in operation is HARPS, operated by ESO in La Silla Observatory/Chile. The Doppler measurements obtained with HARPS are typically obtained using the Cross-Correlation Function technique (CCF). It consists of multiplying the stellar spectrum with a weighted binary mask and finding the minimum of such product as a function of the Doppler shift. It is known that CCF is suboptimal in exploiting the Doppler information in the stellar spectrum. Here, we describe an algorithm to obtain precision RV measurements using least-squares matching of each observed spectrum to a high signal-to-noise ratio template derived from the same observations. Such algorithm is implemented in our software called HARPS-TERRA (Template Enhanced Radial velocity Re-analysis Application). New radial velocity measurements on a representative sample of stars observed by HARPS is used to illustrate the benefits of the proposed method. We show that, compared to CCF, template matching provides a significant improvement in accuracy, specially when applied to M dwarfs.

On the nature of the Herbig B[e] star binary system V921 Scorpii: Geometry and kinematics of the circumprimary disk on sub-AU scales [Replacement]

astro-ph — Fri, 20 Apr 2012 00:32:47 +0000

V921 Scorpii is a close binary system (separation 0.025″) showing the B[e]-phenomenon. The system is surrounded by an enigmatic bipolar nebula, which might have been shaped by episodic mass-loss events, possibly triggered by dynamical interactions between the companion and the circumprimary disk (Kraus et al. 2012a). In this paper, we investigate the spatial structure and kinematics of the circumprimary disk, with the aim to obtain new insights into the still strongly debated evolutionary stage. For this purpose, we combine, for the first time, infrared spectro-interferometry (VLTI/AMBER, R=12,000) and spectro-astrometry (VLT/CRIRES, R=100,000), which allows us to study the AU-scale distribution of circumstellar gas and dust with an unprecedented velocity resolution of 3 km*s^-1. Using a model-independent photocenter analysis technique, we find that the Br-gamma-line emission rotates in the same plane as the dust disk. We can reproduce the wavelength-differential visibilities and phases and the double-peaked line profile using a Keplerian-rotating disk model. The derived mass of the central star is 5.4+/-0.4 M_sun*(d/1150 pc), which is considerably lower than expected from the spectral classification, suggesting that V921 Sco might be more distant (d approx 2kpc) than commonly assumed. Using the geometric information provided by our Br-gamma spectro-interferometric data and Paschen, Brackett, and Pfund line decrement measurements in 61 hydrogen recombination line transitions, we derive the density of the line-emitting gas (N_e=2…6*10^19 m^-3). Given that our measurements can be reproduced with a Keplerian velocity field without outflowing velocity component and the non-detection of age-indicating spectroscopic diagnostics, our study provides new evidence for the pre-main-sequence nature of V921 Sco.

Image Analysis for Cosmology: Shape Measurement Challenge Review & Results from the Mapping Dark Matter Challenge

astro-ph — Thu, 19 Apr 2012 00:42:57 +0000

In this paper we present results from the Mapping Dark Matter competition that expressed the weak lensing shape measurement task in its simplest form and as a result attracted over 700 submissions in 2 months and a factor of 3 improvement in shape measurement accuracy on high signal to noise galaxies, over previously published results, and a factor 10 improvement over methods tested on constant shear blind simulations. We also review weak lensing shape measurement challenges, including the Shear TEsting Programmes (STEP1 and STEP2) and the GRavitational lEnsing Accuracy Testing competitions (GREAT08 and GREAT10).

IVOA Recommendation: SAMP – Simple Application Messaging Protocol Version 1.3 [Replacement]

astro-ph — Thu, 19 Apr 2012 00:37:15 +0000

SAMP is a messaging protocol that enables astronomy software tools to interoperate and communicate. IVOA members have recognised that building a monolithic tool that attempts to fulfil all the requirements of all users is impractical, and it is a better use of our limited resources to enable individual tools to work together better. One element of this is defining common file formats for the exchange of data between different applications. Another important component is a messaging system that enables the applications to share data and take advantage of each other’s functionality. SAMP builds on the success of a prior messaging protocol, PLASTIC, which has been in use since 2006 in over a dozen astronomy applications and has proven popular with users and developers. It is also intended to form a framework for more general messaging requirements.

Gamma-ray burst afterglow broadband fitting based directly on hydrodynamics simulations [Replacement]

astro-ph — Thu, 19 Apr 2012 00:36:48 +0000

We present a powerful new tool for fitting broadband gamma-ray burst afterglow data, which can be used to determine the burst explosion parameters and the synchrotron radiation parameters. By making use of scale invariance between relativistic jets of different energies and different circumburst medium densities, and by capturing the output of high-resolution two-dimensional relativistic hydrodynamical (RHD) jet simulations in a concise summary, the jet dynamics are generated quickly. Our method calculates the full light curves and spectra using linear radiative transfer sufficiently fast to allow for a direct iterative fit of RHD simulations to the data. The fit properly accounts for jet features that so far have not been successfully modeled analytically, such as jet decollimation, inhomogeneity along the shock front and the transitory phase between the early time relativistic and late time non-relativistic outflow. As a first application of the model we simultaneously fit the radio, X-ray and optical data of GRB 990510. We not only find noticeable differences between our findings for the explosion and radiation parameters and those of earlier authors, but also an improved model fit when we include the observer angle in the data fit. The fit method will be made freely available on request and on-line at http://cosmo.nyu.edu/afterglowlibrary. In addition to data fitting, the software tools can also be used to quickly generate a light curve or spectrum for arbitrary observer position, jet and radiation parameters.

Year 3 LUNAR Annual Report to the NASA Lunar Science Institute [Replacement]

astro-ph — Thu, 19 Apr 2012 00:32:32 +0000

The Lunar University Network for Astrophysics Research (LUNAR) is a team of researchers and students at leading universities, NASA centers, and federal research laboratories undertaking investigations aimed at using the Moon as a platform for space science. LUNAR research includes Lunar Interior Physics & Gravitation using Lunar Laser Ranging (LLR), Low Frequency Cosmology and Astrophysics (LFCA), Planetary Science and the Lunar Ionosphere, Radio Heliophysics, and Exploration Science. The LUNAR team is exploring technologies that are likely to have a dual purpose, serving both exploration and science. There is a certain degree of commonality in much of LUNAR’s research. Specifically, the technology development for a lunar radio telescope involves elements from LFCA, Heliophysics, Exploration Science, and Planetary Science; similarly the drilling technology developed for LLR applies broadly to both Exploration and Lunar Science.

Optical Intensity Interferometry with the Cherenkov Telescope Array

astro-ph — Wed, 18 Apr 2012 01:05:38 +0000

With its unprecedented light-collecting area for night-sky observations, the Cherenkov Telescope Array (CTA) holds great potential for also optical stellar astronomy, in particular as a multi-element intensity interferometer for realizing imaging with sub-milliarcsecond angular resolution. Such an order-of-magnitude increase of the spatial resolution achieved in optical astronomy will reveal the surfaces of rotationally flattened stars with structures in their circumstellar disks and winds, or the gas flows between close binaries. Image reconstruction is feasible from the second-order coherence of light, measured as the temporal correlations of arrival times between photons recorded in different telescopes. This technique (once pioneered by Hanbury Brown and Twiss) connects telescopes only with electronic signals and is practically insensitive to atmospheric turbulence and to imperfections in telescope optics. Detector and telescope requirements are very similar to those for imaging air Cherenkov observatories, the main difference being the signal processing (calculating cross correlations between single camera pixels in pairs of telescopes). Observations of brighter stars are not limited by sky brightness, permitting efficient CTA use during also bright-Moon periods. While other concepts have been proposed to realize kilometer-scale optical interferometers of conventional amplitude (phase-) type, both in space and on the ground, their complexity places them much further into the future than CTA, which thus could become the first kilometer-scale optical imager in astronomy.

Stochastic variability in X-ray emission from the black hole binary GRS 1915+105

astro-ph — Wed, 18 Apr 2012 00:52:43 +0000

We examine stochastic variability in the dynamics of X-ray emission from the black hole system GRS 1915+105, a strongly variable microquasar commonly used for studying relativistic jets and the physics of black hole accretion. The analysis of sample observations for 13 different states in both soft (low) and hard (high) energy bands is performed by flicker-noise spectroscopy (FNS), a phenomenological time series analysis method operating on structure functions and power spectrum estimates. We find the values of FNS parameters, including the Hurst exponent, flicker-noise parameter, and characteristic time scales, for each observation based on multiple 2,500-second continuous data segments. We identify four modes of stochastic variability driven by dissipative processes that may be related to viscosity fluctuations in the accretion disk around the black hole: random (RN), power-law (1F), one-scale (1S), and two-scale (2S). The variability modes are generally the same in soft and hard energy bands of the same observation. We discuss the potential for future FNS studies of accreting black holes.

On the sensitivity of closure phases to faint companions in optical long baseline interferometry

astro-ph — Wed, 18 Apr 2012 00:51:47 +0000

We explore the sensitivity and completeness of long baseline interferometric observations for detecting unknown, faint companions around bright unresolved stars. We derive a linear expression for the closure phase signature of a faint companion in the high contrast regime (<0.1), and provide a quantitative estimation of the detection efficiency for the currently offered four-telescope configurations at the Very Large Telescope Interferometer. The results are compared to the performances provided by linear and Y-shaped interferometric configurations in order to identify the ideal array. We find that all configurations have a similar efficiency in discovering companions wider than 10mas. Assuming a closure phase accuracy of 0.25deg, that is typical of state-of-the-art instruments, we predict a median dynamic range of up to six magnitudes when stacking observations obtained at five different hour angles. Surveying bright stars to search for faint companions can be considered as an ideal filler programme for modern interferometric facilities because that places few constraints on the choice of the interferometric configuration.

Compact source detection in multi-channel microwave surveys: from SZ clusters to polarized sources

astro-ph — Wed, 18 Apr 2012 00:44:27 +0000

In this paper we describe the state-of-the art status of multi-frequency detection techniques for compact sources in microwave astronomy. From the simplest cases where the spectral behaviour is well-known (i.e. thermal SZ clusters) to the more complex cases where there is little a priori information (i.e. polarized radio sources) we will review the main advances and the most recent results in the detection problem.

Radio Pulsar Style Timing of Eclipsing Binary Stars from the ASAS Catalogue

astro-ph — Wed, 18 Apr 2012 00:44:10 +0000

The Light-Time Effect (LTE) is observed whenever the distance between the observer and any kind of periodic event changes in time. The usual cause of this distance change is the reflex motion about the system’s barycenter due to the gravitational influence of one or more additional bodies. We analyze 5032 eclipsing contact (EC) and detached (ED) binaries from the All Sky Automated Survey (ASAS) catalogue to detect variations in the times of eclipses which possible can be due to the LTE effect. To this end we use an approach known from the radio pulsar timing where a template radio pulse of a pulsar is used as a reference to measure the times of arrivals of the collected pulses. In our analysis as a template for a photometric time series from ASAS, we use a best-fitting trigonometric series representing the light curve of a given EC or ED. Subsequently, an O-C diagram is built by comparing the template light curve with light curves obtained from subsets of a given time series. Most of the variations we detected in O-Cs correspond to a linear period change. Three show evidence of more than one complete LTE-orbit. For these objects we obtained preliminary orbital solutions. Our results demonstrate that the timing analysis employed in radio pulsar timing can be effectively used to study large data sets from photometric surveys.

Millimeter-wave bolometer array receiver for the Atacama pathfinder experiment Sunyaev-Zel’dovich (APEX-SZ) instrument [Replacement]

astro-ph — Wed, 18 Apr 2012 00:39:21 +0000

The Atacama pathfinder experiment Sunyaev-Zel’dovich (APEX-SZ) instrument is a millimeter-wave cryogenic receiver designed to observe galaxy clusters via the Sunyaev-Zel’dovich effect from the 12 m APEX telescope on the Atacama plateau in Chile. The receiver contains a focal plane of 280 superconducting transition-edge sensor (TES) bolometers instrumented with a frequency-domain multiplexed readout system. The bolometers are cooled to 280 mK via a three-stage helium sorption refrigerator and a mechanical pulse-tube cooler. Three warm mirrors, two 4 K lenses, and a horn array couple the TES bolometers to the telescope. APEX-SZ observes in a single frequency band at 150 GHz with 1′ angular resolution and a 22′ field-of-view, all well suited for cluster mapping. The APEX-SZ receiver has played a key role in the introduction of several new technologies including TES bolometers, the frequency-domain multiplexed readout, and the use of a pulse-tube cooler with bolometers. As a result of these new technologies, the instrument has a higher instantaneous sensitivity and covers a larger field-of-view than earlier generations of Sunyaev-Zel’dovich instruments. The TES bolometers have a median sensitivity of 890 uKcmb-sqrt(s) (NEy of 3.5e-4 sqrt(s)). We have also demonstrated upgraded detectors with improved sensitivity of 530 uKcmb-sqrt(s) (NEy of 2.2e-4 sqrt(s)). Since its commissioning in April 2007, APEX-SZ has been used to map 48 clusters. We describe the design of the receiver and its performance when installed on the APEX telescope.

Dust SEDs in the era of Herschel and Planck: a Hierarchical Bayesian fitting technique [Replacement]

astro-ph — Wed, 18 Apr 2012 00:33:49 +0000

We present a hierarchical Bayesian method for fitting infrared spectral energy distributions (SEDs) of dust emission to observed fluxes. Under the standard assumption of optically thin single temperature (T) sources the dust SED as represented by a power–law modified black body is subject to a strong degeneracy between T and the spectral index beta. The traditional non-hierarchical approaches, typically based on chi-square minimization, are severely limited by this degeneracy, as it produces an artificial anti-correlation between T and beta even with modest levels of observational noise. The hierarchical Bayesian method rigorously and self-consistently treats measurement uncertainties, including calibration and noise, resulting in more precise SED fits. As a result, the Bayesian fits do not produce any spurious anti-correlations between the SED parameters due to measurement uncertainty. We demonstrate that the Bayesian method is substantially more accurate than the chi-square fit in recovering the SED parameters, as well as the correlations between them. As an illustration, we apply our method to Herschel and sub millimeter ground-based observations of the star-forming Bok globule CB244. This source is a small, nearby molecular cloud containing a single low-mass protostar and a starless core. We find that T and beta are weakly positively correlated — in contradiction with the chi-square fits, which indicate a T-beta anti-correlation from the same data-set. Additionally, in comparison to the chi-square fits the Bayesian SED parameter estimates exhibit a reduced range in values.

The EoR Sensitivity of the 128 Antenna Murchison Widefield Array [Replacement]

astro-ph — Wed, 18 Apr 2012 00:31:23 +0000

Using the planned antenna locations of the 128 antenna buildout of the Murchison Widefield Array (MWA), we accurately calculate its sensitivity to the Epoch of Reionization (EoR) power spectrum of redshifted 21 cm emission. Our calculation takes into account synthesis rotation, chromatic and asymmetrical baseline effects, and excludes modes that will be contaminated by foreground subtraction. With one full season of observation on two fields (900 and 700 hours), the MWA will be capable of a 14$\sigma$ detection of the EoR signal along with slope constraints.

A pilgrimage to gravity on GPUs

astro-ph — Tue, 17 Apr 2012 01:13:37 +0000

In this short review we present the developments over the last 5 decades that have led to the use of Graphics Processing Units (GPUs) for astrophysical simulations. Since the introduction of NVIDIA’s Compute Unified Device Architecture (CUDA) in 2007 the GPU has become a valuable tool for N-body simulations and is so popular these days that almost all papers about high precision N-body simulations use methods that are accelerated by GPUs. With the GPU hardware becoming more advanced and being used for more advanced algorithms like gravitational tree-codes we see a bright future for GPU like hardware in computational astrophysics.

The EoR Sensitivity of the 128 Antenna Murchison Widefield Array

astro-ph — Tue, 17 Apr 2012 01:12:48 +0000

Technology developments for a scalable heterodyne MMIC array at W-band

astro-ph — Tue, 17 Apr 2012 01:12:00 +0000

We report on the development of W-band (75-110 GHz) heterodyne receiver technology for large-format astronomical arrays. The receiver system is designed to be both mass-producible, so that the designs could be scaled to thousands of receiver elements, and modular. Most of the receiver function- ality is integrated into compact Monolithic Microwave Integrated Circuit (MMIC) amplifier-based multichip modules. The MMIC modules include a chain of InP MMIC low-noise amplifiers, coupled-line bandpass filters and sub-harmonic Schottky diode mixers. The receiver signals will be routed to and from the MMIC modules on a multilayer high frequency laminate, which includes splitters, amplifiers, and frequency doublers. A prototype MMIC module has exhibited a band-averaged noise temperature of 41 K from 82-100 GHz and a gain of 29 dB at 15 K, which is the state- of-the-art for heterodyne multi-chip modules.

A New Fast Silicon Photomultiplier Photometer

astro-ph — Tue, 17 Apr 2012 01:05:48 +0000

The realization of low-cost instruments with high technical performance is a goal which deserves some efforts in an epoch of fast technological developments: indeed such instruments can be easily reproduced and therefore allow to open new research programs in several Observatories. We realized a fast optical photometer based on the SiPM technology, using commercially available modules. Using low-cost components we have developed a custom electronic chain to extract the signal produced by a commercial MPPC module produced by Hamamatsu, in order to obtain sub millisecond sampling of the light curve of astronomical sources, typically pulsars. In the early February 2011 we observed the Crab Pulsar at the Cassini telescope with our prototype photometer, deriving its period, power spectrum and shape of its light curve in very good agreement with the results obtained in the past with other instruments.

Investigating operation of the Internet in orbit: Five years of collaboration around CLEO [Cross-Listing]

astro-ph — Tue, 17 Apr 2012 01:03:40 +0000

The Cisco router in Low Earth Orbit (CLEO) was launched into space as an experimental secondary payload onboard the UK Disaster Monitoring Constellation (UK-DMC) satellite in September 2003. The UK-DMC satellite is one of an increasing number of DMC satellites in orbit that rely on the Internet Protocol (IP) for command and control and for delivery of data from payloads. The DMC satellites, built by Surrey Satellite Technology Ltd (SSTL), have imaged the effects of Hurricane Katrina, the Indian Ocean Tsunami, and other events for disaster relief under the International Space and Major Disasters Charter. It was possible to integrate the Cisco mobile access router into the UK-DMC satellite as a result of the DMC satellites’ adoption of existing commercial networking standards, using IP over Frame Relay over standard High-Level Data Link Control, or HDLC (ISO 13239) on standard serial interfaces. This approach came from work onboard SSTL’s earlier UoSAT-12 satellite

Saratoga: scalable, speedy data delivery for sensor networks [Cross-Listing]

astro-ph — Tue, 17 Apr 2012 01:03:23 +0000

A networking transport protocol, named Saratoga, has been developed at the University of Surrey for efficient delivery of imagery from Internet-Protocol-based remote-sensing satellites. Saratoga is now being implemented and evaluated for use for the high-end data-delivery needs of astronomers using large, advanced, radio telescopes. These telescopes are expected to take advantage of Internet technologies. This brief paper outlines the reasons for the creation and adoption of this protocol, discusses how it differs from and complements other protocols, and summarises the worldwide collaboration that is making this development possible.

SaVi: satellite constellation visualization

astro-ph — Tue, 17 Apr 2012 01:03:09 +0000

SaVi, a program for visualizing satellite orbits, movement, and coverage, is maintained at the University of Surrey. This tool has been used for research in academic papers, and by industry companies designing and intending to deploy satellite constellations. It has also proven useful for demonstrating aspects of satellite constellations and their geometry, coverage and movement for educational and teaching purposes. SaVi is introduced and described briefly here.

Year 3 LUNAR Annual Report to the NASA Lunar Science Institute

astro-ph — Tue, 17 Apr 2012 00:45:04 +0000

The AAVSO 2011 Demographic and Background Survey

astro-ph — Tue, 17 Apr 2012 00:44:46 +0000

In 2011, the AAVSO conducted a survey of 615 people who are or were recently active in the 101-year old organization. The survey included questions about their demographic background and variable star interests. Data are descriptively analyzed and compared with prior surveys. Results show an organization of very highly educated, largely male amateur and professional astronomers distributed across 108 countries. Participants tend to be loyal, with the average time of involvement in the AAVSO reported as 14 years. Most major demographic factors have not changed much over time. However, the average age of new members is increasing. Also, a significant portion of the respondents report being strictly active in a non-observing capacity, reflecting the growing mission of the organization. Motivations of participants are more aligned with scientific contribution than with that reported by other citizen science projects. This may help explain why a third of all respondents are an author or co-author of a paper in an astronomical journal. Finally, there is some evidence that participation in the AAVSO has a greater impact on the respondents’ view of their role in astronomy compared to that expected through increasing amateur astronomy experience alone. Results paint a picture of participants in a modern, advanced citizen science organization.

Modelling the reflective thermal contribution to the acceleration of the Pioneer spacecraft [Replacement]

astro-ph — Tue, 17 Apr 2012 00:40:08 +0000

We present an improved method to compute the radiative momentum transfer in the Pioneer 10 & 11 spacecraft that takes into account both diffusive and specular reflection. The method allows for more reliable results regarding the thermal acceleration of the deep-space probes, confirming previous findings. A parametric analysis is performed in order to set an upper and lower-bound for the thermal acceleration and its evolution with time.

Detection noise bias and variance in the power spectrum and bispectrum in optical interferometry [Replacement]

astro-ph — Tue, 17 Apr 2012 00:39:57 +0000

Long-baseline optical interferometry uses the power spectrum and bispectrum constructs as fundamental observables. Noise arising in the detection of the fringe pattern gives rise to both variance and biases in the power spectrum and bispectrum. Previous work on correcting the biases and estimating the variances for these quantities typically includes restrictive assumptions about the sampling of the interferogram and/or about the relative importance of Poisson and Gaussian noise sources. Until now it has been difficult to accurately compensate for systematic biases in data which violates these assumptions. We seek a formalism to allow the construction of bias-free estimators of the bispectrum and power spectrum, and to estimate their variances, under less restrictive conditions which include both unevenly-sampled data and measurements affected by a combination of noise sources with Poisson and Gaussian statistics. We used a method based on the moments of the noise distributions to derive formulae for the biases introduced to the power spectrum and bispectrum when the complex fringe amplitude is derived from an arbitrary linear combinations of a set of discrete interferogram measurements. We simulated interferograms with different combinations of photon noise and read noise and with different fringe encoding schemes to illustrate the effects of these biases. We have derived formulae for bias-free estimators of the power spectrum and bispectrum which can be used with any linear estimator of the fringe complex amplitude. We have demonstrated the importance of bias-free estimators for the case of the detection of faint companions (for example exoplanets) using closure phase nulling. We have derived formulae for the variance of the power spectrum and have shown how the variance of the bispectrum could be calculated.

Studies of a three-stage dark matter and neutrino observatory based on multi-ton combinations of liquid xenon and liquid argon detectors [Replacement]

astro-ph — Tue, 17 Apr 2012 00:38:59 +0000

We study a three stage dark matter and neutrino observatory based on multi-ton two-phase liquid Xe and Ar detectors with sufficiently low backgrounds to be sensitive to WIMP dark matter interaction cross sections down to 10E-47 cm^2, and to provide both identification and two independent measurements of the WIMP mass through the use of the two target elements in a 5:1 mass ratio, giving an expected similarity of event numbers. The same detection systems will also allow measurement of the pp solar neutrino spectrum, the neutrino flux and temperature from a Galactic supernova, and neutrinoless double beta decay of 136Xe to the lifetime level of 10E27 – 10E28 y corresponding to the Majorana mass predicted from current neutrino oscillation data. The proposed scheme would be operated in three stages G2, G3, G4, beginning with fiducial masses 1-ton Xe + 5-ton Ar (G2), progressing to 10-ton Xe + 50-ton Ar (G3) then, dependent on results and performance of the latter, expandable to 100-ton Xe + 500-ton Ar (G4). This method of scale-up offers the advantage of utilizing the Ar vessel and ancillary systems of one stage for the Xe detector of the succeeding stage, requiring only one new detector vessel at each stage. Simulations show the feasibility of reducing or rejecting all external and internal background levels to a level <1 events per year for each succeeding mass level, by utilizing an increasing outer thickness of target material as self-shielding. The system would, with increasing mass scale, become increasingly sensitive to annual signal modulation, the agreement of Xe and Ar results confirming the Galactic origin of the signal. Dark matter sensitivities for spin-dependent and inelastic interactions are also included, and we conclude with a discussion of possible further gains from the use of Xe/Ar mixtures.

AAO Observer Number 121 (February 2012)

astro-ph — Mon, 16 Apr 2012 00:46:36 +0000

The newsletter of the Australian Astronomical Observatory. In this issue: Using 2dF and AAOmega to Harness the Full Power of the Supernova Legacy Survey; Emission Lines in the Near Infrared: Tracing the Violent ISM; Dancing Starbugs: vacuum adhesion, field rotation and other progress; A message of progress from HERMES; Imaging with the 2dF Focal Plane Imager; and all the usual columns and news from the Observatory.

IVOA Recommendation: Spectrum Data Model 1.1

astro-ph — Mon, 16 Apr 2012 00:40:04 +0000

We present a data model describing the structure of spectrophotometric datasets with spectral and temporal coordinates and associated metadata. This data model may be used to represent spectra, time series data, segments of SED (Spectral Energy Distributions) and other spectral or temporal associations.

Interferometric Observations of Rapidly Rotating Stars

astro-ph — Fri, 13 Apr 2012 00:51:24 +0000

Optical interferometry provides us with a unique opportunity to improve our understanding of stellar structure and evolution. Through direct observation of rotationally distorted photospheres at sub-milliarcsecond scales, we are now able to characterize latitude dependencies of stellar radius, temperature structure, and even energy transport. These detailed new views of stars are leading to revised thinking in a broad array of associated topics, such as spectroscopy, stellar evolution, and exoplanet detection. As newly advanced techniques and instrumentation mature, this topic in astronomy is poised to greatly expand in depth and influence.

A close halo of large transparent grains around extreme red giant stars

astro-ph — Fri, 13 Apr 2012 00:47:36 +0000

Intermediate-mass stars end their lives by ejecting the bulk of their envelope via a slow dense wind back into the interstellar medium, to form the next generation of stars and planets. Stellar pulsations are thought to elevate gas to an altitude cool enough for the condensation of dust, which is then accelerated by radiation pressure from starlight, entraining the gas and driving the wind. However accounting for the mass loss has been a problem due to the difficulty in observing tenuous gas and dust tens of milliarcseconds from the star, and there is accordingly no consensus on the way sufficient momentum is transferred from the starlight to the outflow. Here, we present spatially-resolved, multi-wavelength observations of circumstellar dust shells of three stars on the asymptotic giant branch of the HR diagram. When imaged in scattered light, dust shells were found at remarkably small radii (<~ 2 stellar radii) and with unexpectedly large grains (~300 nm radius). This proximity to the photosphere argues for dust species that are transparent to starlight and therefore resistant to sublimation by the intense radiation field. While transparency usually implies insufficient radiative pressure to drive a wind, the radiation field can accelerate these large grains via photon scattering rather than absorption – a plausible mass-loss mechanism for lower-amplitude pulsating stars.

Design Concepts for the Cherenkov Telescope Array [Replacement]

astro-ph — Fri, 13 Apr 2012 00:38:00 +0000

Ground-based gamma-ray astronomy has had a major breakthrough with the impressive results obtained using systems of imaging atmospheric Cherenkov telescopes. Ground-based gamma-ray astronomy has a huge potential in astrophysics, particle physics and cosmology. CTA is an international initiative to build the next generation instrument, with a factor of 5-10 improvement in sensitivity in the 100 GeV to 10 TeV range and the extension to energies well below 100 GeV and above 100 TeV. CTA will consist of two arrays (one in the north, one in the south) for full sky coverage and will be operated as open observatory. The design of CTA is based on currently available technology. This document reports on the status and presents the major design concepts of CTA.

Bonsai: A GPU Tree-Code

astro-ph — Thu, 12 Apr 2012 00:50:59 +0000

We present a gravitational hierarchical N-body code that is designed to run efficiently on Graphics Processing Units (GPUs). All parts of the algorithm are executed on the GPU which eliminates the need for data transfer between the Central Processing Unit (CPU) and the GPU. Our tests indicate that the gravitational tree-code outperforms tuned CPU code for all parts of the algorithm and show an overall performance improvement of more than a factor 20, resulting in a processing rate of more than 2.8 million particles per second.

Terahertz hot electron bolometer waveguide mixers for GREAT

astro-ph — Thu, 12 Apr 2012 00:46:28 +0000

Supplementing the publications based on the first-light observations with the German Receiver for Astronomy at Terahertz frequencies (GREAT) on SOFIA, we present background information on the underlying heterodyne detector technology. We describe the superconducting hot electron bolometer (HEB) detectors that are used as frequency mixers in the L1 (1400 GHz), L2 (1900 GHz), and M (2500 GHz) channels of GREAT. Measured performance of the detectors is presented and background information on their operation in GREAT is given. Our mixer units are waveguide-based and couple to free-space radiation via a feedhorn antenna. The HEB mixers are designed, fabricated, characterized, and flight-qualified in-house. We are able to use the full intermediate frequency bandwidth of the mixers using silicon-germanium multi-octave cryogenic low-noise amplifiers with very low input return loss. Superconducting HEB mixers have proven to be practical and sensitive detectors for high-resolution THz frequency spectroscopy on SOFIA. We show that our niobium-titanium-nitride (NbTiN) material HEBs on silicon nitride (SiN) membrane substrates have an intermediate frequency (IF) noise roll-off frequency above 2.8 GHz, which does not limit the current receiver IF bandwidth. Our mixer technology development efforts culminate in the first successful operation of a waveguide-based HEB mixer at 2.5 THz and deployment for radioastronomy. A significant contribution to the success of GREAT is made by technological development, thorough characterization and performance optimization of the mixer and its IF interface for receiver operation on SOFIA. In particular, the development of an optimized mixer IF interface contributes to the low passband ripple and excellent stability, which GREAT demonstrated during its initial successful astronomical observation runs.

New Kernels for SPH

astro-ph — Thu, 12 Apr 2012 00:43:53 +0000

We investigate the suitability of the Wendland functions as smoothing kernels for smoothed particle hydrodynamics (SPH) and compare them with the traditional B-splines. Linear stability analysis in three dimensions and test simulations demonstrate that the Wendland kernels avoid the clumping (or pairing) instability for all neighbour numbers NH, despite having vanishing derivative at the origin. This disproves traditional ideas about the origin of this instability. Instead, we give an explanation based on the kernel Fourier transform, but also an interpretation in terms of the SPH density estimator. The Wendland kernels are computationally more convenient than the higher-order B-splines and thus allow large NH, which we show are required to obtain decent numerical accuracy for strongly shearing flows (note that computational costs rise sub-linear with NH). At low NH the quartic B-spline kernel with NH = 60 obtains much better convergence then the standard cubic B-spline with NH<=57.

Support for the thermal origin of the Pioneer anomaly [Cross-Listing]

astro-ph — Thu, 12 Apr 2012 00:42:57 +0000

We investigate the possibility that the anomalous acceleration of the Pioneer 10 and 11 spacecraft is due to the recoil force associated with an anisotropic emission of thermal radiation off the vehicles. To this end, relying on the project and spacecraft design documentation, we constructed a comprehensive finite-element thermal model of the two spacecraft. Then, we numerically solve thermal conduction and radiation equations using the actual flight telemetry as boundary conditions. We use the results of this model to evaluate the effect of the thermal recoil force on the Pioneer 10 spacecraft at various heliocentric distances. We found that the magnitude, temporal behavior, and direction of the resulting thermal acceleration are all similar to the properties of the observed anomaly. As a novel element of our investigation, we develop a parameterized model for the thermal recoil force and estimate the coefficients of this model independently from navigational Doppler data. We find no statistically significant difference between the two estimates and conclude that once the thermal recoil force is properly accounted for, no anomalous acceleration remains.

Three-dimensional track reconstruction for directional Dark Matter detection [Replacement]

astro-ph — Thu, 12 Apr 2012 00:34:39 +0000

Directional detection of Dark Matter is a promising search strategy. However, to perform such detection, a given set of parameters has to be retrieved from the recoiling tracks : direction, sense and position in the detector volume. In order to optimize the track reconstruction and to fully exploit the data of forthcoming directional detectors, we present a likelihood method dedicated to 3D track reconstruction. This new analysis method is applied to the MIMAC detector. It requires a full simulation of track measurements in order to compare real tracks to simulated ones. We conclude that a good spatial resolution can be achieved, i.e. sub-mm in the anode plane and cm along the drift axis. This opens the possibility to perform a fiducialization of directional detectors. The angular resolution is shown to range between 20$^\circ$ to 80$^\circ$, depending on the recoil energy, which is however enough to achieve a high significance discovery of Dark Matter. On the contrary, we show that sense recognition capability of directional detectors depends strongly on the recoil energy and the drift distance, with small efficiency values (50%-70%). We suggest not to consider this information either for exclusion or discovery of Dark Matter for recoils below 100 keV and then to focus on axial directional data.

On the nature of the Herbig B[e] star binary system V921 Scorpii: Geometry and kinematics of the circumprimary disk on sub-AU scales

astro-ph — Wed, 11 Apr 2012 00:53:17 +0000

V921 Scorpii is a close binary system (separation 0.025″) showing the B[e]-phenomenon. The system is surrounded by an enigmatic bipolar nebula, which might have been shaped by episodic mass-loss events, possibly triggered by dynamical interactions between the companion and the circumprimary disk (Kraus et al. 2012a). In this paper, we investigate the spatial structure and kinematics of the circumprimary disk, with the aim to obtain new insights into the still strongly debated evolutionary stage. For this purpose, we combine, for the first time, infrared spectro-interferometry (VLTI/AMBER, R=12,000) and spectro-astrometry (VLT/CRIRES, R=100,000), which allows us to study the AU-scale distribution of circumstellar gas and dust with an unprecedented velocity resolution of 3 km…^-1. Using a model-independent photocenter analysis technique, we find that the Br{\gamma}-line emission rotates in the same plane as the dust disk. We can reproduce the wavelength-differential visibilities and phases and the double-peaked line profile using a Keplerian-rotating disk model. The derived mass of the central star is 5.4+/-0.4 M_sun\cdot(d/1150 pc), which is considerably lower than expected from the spectral classification, suggesting that V921 Sco might be more distant (d approx 2kpc) than commonly assumed. Using the geometric information provided by our Br-gamma spectro-interferometric data and Paschen, Brackett, and Pfund line decrement measurements in 61 hydrogen recombination line transitions, we derive the density of the line-emitting gas (N_e=2…6\cdot10^19 m^-3). Given that our measurements can be reproduced with a Keplerian velocity field without outflowing velocity component and the non-detection of age-indicating spectroscopic diagnostics, our study provides new evidence for the pre-main-sequence nature of V921 Sco.

Numerical simulation of time delay interferometry for NGO/eLISA [Cross-Listing]

astro-ph — Wed, 11 Apr 2012 00:42:03 +0000

NGA/eLISA is a new mission proposal with arm length 106 km and one interferometer down-scaled from LISA (http://elisa-ngo.org/). Just like LISA and ASTROD-GW, in order to attain the requisite sensitivity for NGO/eLISA, laser frequency noise must be suppressed below the secondary noises such as the optical path noise, acceleration noise etc. In previous papers, we have used the CGC 2.7 ephemeris to numerically simulate the time delay interferometry for LISA and ASTROD-GW with one arm dysfunctional and found that they are both well below their respective limits under which the laser frequency noise is required to be suppressed. In this paper, we follow the same procedure to simulate the time delay interferometry numerically. To do this, we work out a set of 1000-day optimized mission orbits of NGO/eLISA spacecraft starting at January 1st, 2021 using the CGC 2.7 ephemeris framework. We then use this numerical solution to calculate the residual optical path differences in the second-generation solutions of our previous papers. The maximum path length difference, for all configuration calculated, is below 12 mm (40 ps). This is well below the limit under which the laser frequency noise is required to be suppressed for NGO/eLISA. We compare and discuss the resulting differences due to different arm lengths for various mission proposals — NGO/eLISA, an NGO-LISA-type mission with a nominal arm length of 2 \times 10^6 km, LISA and ASTROD-GW.

A Simple Method to Test for Energy-Dependent Dispersion in High Energy Light-Curves of Astrophysical Sources

astro-ph — Wed, 11 Apr 2012 00:40:30 +0000

In this paper we discuss a simple method of testing for the presence of energy-dependent dispersion in high energy data-sets. It uses the minimisation of the Kolmogorov distance between the cumulative distribution of two probability functions as the statistical metric to estimate the magnitude of any spectral dispersion within transient features in a light-curve and we also show that it performs well in the presence of modest energy resolutions (~20%) typical of gamma-ray observations. After presenting the method in detail we apply it to a parameterised simulated lightcurve based on the extreme VHE gamma-ray flare of PKS 2155-304 observed with H.E.S.S. in 2006, in order to illustrate its potential through the concrete example of setting constraints on quantum-gravity induced Lorentz invariance violation (LIV) effects. We obtain comparable limits to those of the most advanced techniques used in LIV searches applied to similar datasets, but the present method has the advantage of being particularly straightforward to use. Whilst the development of the method was motivated by LIV searches, it is also applicable to other astrophysical situations where energy-dependent dispersion is expected, such as spectral lags from the acceleration and cooling of particles in relativistic outflows.

A sparse octree gravitational N-body code that runs entirely on the GPU processor [Replacement]

astro-ph — Wed, 11 Apr 2012 00:37:44 +0000

We present parallel algorithms for constructing and traversing sparse octrees on graphics processing units (GPUs). The algorithms are based on parallel-scan and sort methods. To test the performance and feasibility, we implemented them in CUDA in the form of a gravitational tree-code which completely runs on the GPU.(The code is publicly available at: http://castle.strw.leidenuniv.nl/software.html) The tree construction and traverse algorithms are portable to many-core devices which have support for CUDA or OpenCL programming languages. The gravitational tree-code outperforms tuned CPU code during the tree-construction and shows a performance improvement of more than a factor 20 overall, resulting in a processing rate of more than 2.8 million particles per second.

A generalized likelihood ratio test statistic for Cherenkov telescope data [Replacement]

astro-ph — Wed, 11 Apr 2012 00:34:49 +0000

Astrophysical sources of TeV gamma rays are usually established by Cherenkov telescope observations. These counting type instruments have a field of view of few degrees in diameter and record large numbers of particle air showers via their Cherenkov radiation in the atmosphere. The showers are either induced by gamma rays or diffuse cosmic ray background. The commonly used test statistic to evaluate a possible gamma-ray excess is Li and Ma (1983), Eq. 17, which can be applied to independent on- and off-source observations, or scenarios that can be approximated as such. This formula however is unsuitable if the data are taken in so-called “wobble” mode (pointing to several offset positions around the source), if at the same time the acceptance shape is irregular or even depends on operating parameters such as the pointing direction or telescope multiplicity. To provide a robust test statistic in such cases, this paper explores a possible generalization of the likelihood ratio concept on which the formula of Li and Ma is based. In doing so, the multi-pointing nature of the data and the typically known instrument point spread function are fully exploited to derive a new, semi-numerical test statistic. Due to its flexibility and robustness against systematic uncertainties, it is not only useful for detection purposes, but also for skymapping and source shape fitting. Simplified Monte Carlo simulations are presented to verify the results, and several applications and further generalizations of the concept are discussed.

Evolutionary Computation in Astronomy and Astrophysics: A Review [Replacement]

astro-ph — Wed, 11 Apr 2012 00:33:26 +0000

In general Evolutionary Computation (EC) includes a number of optimization methods inspired by biological mechanisms of evolution. The methods catalogued in this area use the Darwinian principles of life evolution to produce algorithms that returns high quality solutions to hard-to-solve optimization problems. The main strength of EC is precisely that they provide good solutions even if the computational resources (e.g., running time) are limited. Astronomy and Astrophysics are two fields that often require optimizing problems of high complexity or analyzing a huge amount of data and the so-called complete optimization methods are inherently limited by the size of the problem/data. For instance, reliable analysis of large amounts of data is central to modern astrophysics and astronomical sciences in general. EC techniques perform well where other optimization methods are inherently limited (as complete methods applied to NP-hard problems), and in the last ten years, numerous proposals have come up that apply with greater or lesser success methodologies of evolutional computation to common engineering problems. Some of these problems, such as the estimation of non-lineal parameters, the development of automatic learning techniques, the implementation of control systems, or the resolution of multi-objective optimization problems, have had (and have) a special repercussion in the fields. For these reasons EC emerges as a feasible alternative for traditional methods. In this paper, we discuss some promising applications in this direction and a number of recent works in this area; the paper also includes a general description of EC to provide a global perspective to the reader and gives some guidelines of application of EC techniques for future research

Status of the TREND project

astro-ph — Tue, 10 Apr 2012 00:50:40 +0000

The Tianshan Radio Experiment for Neutrino Detection (TREND) is a sino-french collaboration (CNRS/IN2P3 and Chinese Academy of Science) developing an autonomous antenna array for the detection of high energy Extensive Air Showers (EAS) on the site of the 21CMA radio observatory. The autonomous detection and identification of EAS was achieved by TREND on a prototype array in 2009. This result was confirmed soon after when EAS radio-candidates could be tagged as cosmic ray events by an array of particle detectors running in parallel at the same location. This result is an important milestone for TREND, and more generally, for the maturation of the EAS radio-detection technique. The array is presently composed of 50 antennas covering a total area of ~1.2 km^2, running in steady conditions since March 2011. We are presently processing the data to identify EAS radio-candidates. In a long term perspective, TREND is intended to search for high energy tau neutrinos. Here we only report on the results achieved so far by TREND.

Earth magnetic field effects on the cosmic electron flux as background for Cherenkov Telescopes at low energies

astro-ph — Tue, 10 Apr 2012 00:42:46 +0000

Cosmic ray electrons and positrons constitute an important component of the background for imaging atmospheric Cherenkov Telescope Systems with very low energy thresholds. As the primary energy of electrons and positrons decreases, their contribution to the background trigger rate dominates over protons, at least in terms of differential rates against actual energies. After event reconstruction, this contribution might become comparable to the proton background at energies of the order of few GeV. It is well known that the flux of low energy charged particles is suppressed by the Earth’s magnetic field. This effect strongly depends on the geographical location, the direction of incidence of the charged particle and its mass. Therefore, the geomagnetic field can contribute to diminish the rate of the electrons and positrons detected by a given array of Cherenkov Telescopes. In this work we study the propagation of low energy primary electrons in the Earth’s magnetic field by using the backtracking technique. We use a more realistic geomagnetic field model than the one used in previous calculations. We consider some sites relevant for new generations of imaging atmospheric Cherenkov Telescopes. We also study in detail the case of 5@5, a proposed low energy Cherenkov Telescope array.

"WISEASS" – A State-of-the-art Interactive Supernova Spectroscopy Database

astro-ph — Tue, 10 Apr 2012 00:41:54 +0000

We have entered an era of massive data sets in astronomy. In particular, the number of supernova (SN) discoveries and classifications has substantially increased over the years from few tens to thousands per year. It is no longer the case that observations of a few prototypical events encapsulate most spectroscopic information about SNe, motivating the development of modern tools to collect, archive, organize and distribute spectra in general, and SN spectra in particular. For this reason we have developed the Weizmann Institute of Science Experimental Astrophysics Spectroscopy System – WISEASS — an SQL-based database (DB) with an interactive web-based graphical interface. The system serves as an archive of high quality SN spectra, including both historical (legacy) data as well as data that is accumulated by ongoing modern programs. The archive provides information about objects, their spectra, and related meta-data. Utilizing interactive plots, we provide a graphical interface to visualize data, perform line identification of the major relevant species, determine object redshifts, classify SNe and measure expansion velocities. Guest users may view and download spectra or other data that have been placed in the public domain. Registered users may also view and download data that are proprietary to specific programs with which they are associated. The DB currently holds >7700 spectra, of which >4600 are public; the latter include published spectra from the Palomar Transient Factory (PTF), the Caltech-Core-Collapse Program (CCCP), all of the SUSPECT (SUpernova SPECTrum) archive and the CfA Type Ia SN spectral archive. It offers an efficient and convenient way to archive data and share it with colleagues, and we expect that data stored in this way will be easy to access, increasing its visibility, usefulness and scientific impact.

The reinstatement of funding to the Overwhelmingly Large Telescope Project [Replacement]

astro-ph — Tue, 10 Apr 2012 00:32:32 +0000

While space-based telescopes have had a part to play ultimately I feel that it is telescopes like myself that have contributed the most to pushing the boundaries of our knowledge. With current advances in optical engineering we have the ability to create segmented mirrors of considerable size. The scientific possibilities of a single 100m reflecting telescope are considerable, for example, allowing for the detection of biological components in exoplanet atmospheres. One such project was OWL, a project very close to my heart and one that was sadly cancelled before it even began. In light of the breakthroughs that such an instrument could have given the community I recommend the cancellation of the OWL to be reconsidered and for funding to be reinstated to the project.

Atmospheric dispersion effects in weak lensing measurements

astro-ph — Mon, 09 Apr 2012 00:43:28 +0000

The wavelength dependence of atmospheric refraction causes elongation of finite-bandwidth images along the elevation vector, which produces spurious signals in weak gravitational lensing shear measurements unless this atmospheric dispersion is calibrated and removed to high precision. Because astrometric solutions and point spread function (PSF) characteristics are typically calibrated from stellar images, differences between the reference stars’ spectra and the galaxies’ spectra will leave residual errors in both the astrometric positions ($\Delta{\bar{R}}$) and in the second moment (width) of the wavelength-averaged PSF ($\Delta{v}$) for galaxies. We estimate the level of $\Delta{V}$ that will induce spurious weak lensing signals in PSF-corrected galaxy shapes that exceed the statistical errors of the {\em Dark Energy Survey (DES)} and the {\em Large Synoptic Survey Telescope (LSST)} cosmic-shear experiments. We also estimate the $\Delta{\bar{R}}$ signals that will produce unacceptable spurious distortions after stacking of exposures taken at different airmasses and hour angles. Using standard galaxy and stellar spectral templates we calculate the resultant errors in the $griz$ bands, and find that atmospheric dispersion differentials, left uncorrected, exceed the {\em DES} cosmic-shear requirements in the $g$ and $r$ bands, and exceed the stricter LSST requirements in $i$ band. We find that a simple correction linear in galaxy color is accurate enough to recover the use of $r$ band for DES and $i$ band for LSST. More complex approaches to correction of the atmospheric dispersion signal will be needed to use the $g$ band for DES cosmic-shear measurements or to use the $g$ or $r$ bands for LSST cosmic-shear measurements.

NIKEL: Electronics and data acquisition for kilopixels kinetic inductance camera

astro-ph — Mon, 09 Apr 2012 00:38:59 +0000

A prototype of digital frequency multiplexing electronics allowing the real time monitoring of microwave kinetic inductance detector (MKIDs) arrays for mm-wave astronomy has been developed. Thanks to the frequency multiplexing, it can monitor simultaneously 400 pixels over a 500 MHz bandwidth and requires only two coaxial cables for instrumenting such a large array. The chosen solution and the performance achieved are presented in this paper.

Simulation Studies of the Backscattering Signal in HSRL Technique

astro-ph — Fri, 06 Apr 2012 00:49:36 +0000

The technique of High Spectral Resolution Lidar (HSRL) for atmospheric monitoring allows the determination of the aerosol to molecular ratio and can be used in UHECR Observatories using air fluorescence telescopes. By this technique a more accurate estimate of the Cherenkov radiation superimposed to the fluorescence signal can be achieved. A laboratory setup was developed to determine the backscattering coefficients using microparticles diluted in water and diffusion interfaces. In this setup we used a CW SLM laser at 532 nm and a 250 mm Newtonian telescope. Simulations of the above experimental configuration have been made using Scatlab\c{opyright}, FINESSE\c{opyright} 0.99.8 and MATLAB\c{opyright} and are presented in this work. We compare the simulated 2-dimensional Fabry-Perot fringe images of the backscattering signal recorded in the CCD sensor with that of experimental ones. Additionally, we simulated the backscattering of the laser beam by the atmosphere at a height of 2000 m and we have studied the influence of the beam and its diameter on the fringe image.

TeV Gamma-ray Astronomy: A Summary

astro-ph — Fri, 06 Apr 2012 00:40:08 +0000

The field of TeV gamma-ray astronomy has produced many exciting results over the last decade. Both the source catalogue, and the range of astrophysical questions which can be addressed, continue to expand. This article presents a topical review of the field, with a focus on the observational results of the imaging atmospheric Cherenkov telescope arrays. The results encompass pulsars and their nebulae, supernova remnants, gamma-ray binary systems, star forming regions and starburst and active galaxies.

Optimal filters for detecting cosmic bubble collisions [Replacement]

astro-ph — Fri, 06 Apr 2012 00:33:31 +0000

A number of well-motivated extensions of the LCDM concordance cosmological model postulate the existence of a population of sources embedded in the cosmic microwave background (CMB). One such example is the signature of cosmic bubble collisions which arise in models of eternal inflation. The most unambiguous way to test these scenarios is to evaluate the full posterior probability distribution of the global parameters defining the theory; however, a direct evaluation is computationally impractical on large datasets, such as those obtained by the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck. A method to approximate the full posterior has been developed recently, which requires as an input a set of candidate sources which are most likely to give the largest contribution to the likelihood. In this article, we present an improved algorithm for detecting candidate sources using optimal filters, and apply it to detect candidate bubble collision signatures in WMAP 7-year observations. We show both theoretically and through simulations that this algorithm provides an enhancement in sensitivity over previous methods by a factor of approximately two. Moreover, no other filter-based approach can provide a superior enhancement of these signatures. Applying our algorithm to WMAP 7-year observations, we detect eight new candidate bubble collision signatures for follow-up analysis.

Observations of Low Frequency Solar Radio Bursts from the Rosse Solar-Terrestrial Observatory [Replacement]

astro-ph — Fri, 06 Apr 2012 00:31:21 +0000

The Rosse Solar-Terrestrial Observatory (RSTO; www.rosseobservatory.ie) was established at Birr Castle, Co. Offaly, Ireland (53 05′38.9″, 7 55′12.7″) in 2010 to study solar radio bursts and the response of the Earth’s ionosphere and geomagnetic field. To date, three Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Observatory (CALLISTO) spectrometers have been installed, with the capability of observing in the frequency range 10-870 MHz. The receivers are fed simultaneously by biconical and log-periodic antennas. Nominally, frequency spectra in the range 10-400 MHz are obtained with 4 sweeps per second over 600 channels. Here, we describe the RSTO solar radio spectrometer set-up, and present dynamic spectra of a sample of Type II, III and IV radio bursts. In particular, we describe fine-scale structure observed in Type II bursts, including band splitting and rapidly varying herringbone features.

Acoustic Transmitters for Underwater Neutrino Telescopes

astro-ph — Thu, 05 Apr 2012 00:52:01 +0000

In this paper acoustic transmitters that were developed for use in underwater neutrino telescopes are presented. Firstly, an acoustic transceiver has been developed as part of the acoustic positioning system of neutrino telescopes. These infrastructures are not completely rigid and require a positioning system in order to monitor the position of the optical sensors which move due to sea currents. To guarantee a reliable and versatile system, the transceiver has the requirements of reduced cost, low power consumption, high pressure withstanding (up to 500 bars), high intensity for emission, low intrinsic noise, arbitrary signals for emission and the capacity of acquiring and processing received signals. Secondly, a compact acoustic transmitter array has been developed for the calibration of acoustic neutrino detection systems. The array is able to mimic the signature of ultra-high-energy neutrino interaction in emission directivity and signal shape. The technique of parametric acoustic sources has been used to achieve the proposed aim. The developed compact array has practical features such as easy manageability and operation. The prototype designs and the results of different tests are described. The techniques applied for these two acoustic systems are so powerful and versatile that may be of interest in other marine applications using acoustic transmitters.

Development of an acoustic transceiver for positioning systems in Underwater Neutrino Telescopes

astro-ph — Thu, 05 Apr 2012 00:48:10 +0000

In this paper, we present the acoustic transceiver developed for the positioning system in underwater neutrino telescopes. These infrastructures are not completely rigid and need a positioning system in order to monitor the position of the optical sensors of the telescope which have some degree of motion due to sea currents. To have a highly reliable and versatile system in the infrastructure, the transceiver has the requirements of reduced cost, low power consumption, high intensity for emission, low intrinsic noise, arbitrary signals for emission and the capacity of acquiring and processing the received signal on the board. The solution proposed and presented here consists of an acoustic transducer that works in the 20-40 kHz region and withstands high pressures (up to 500 bars). The electronic-board can be configured from shore and is able to feed the transducer with arbitrary signals and to control the transmitted and received signals with very good timing precision. The results of the different tests done on the transceiver in the laboratory are described here, as well as the change implemented for its integration in the Instrumentation Line of ANTARES for the in situ tests. We consider the transceiver design is so versatile that it may be used in other kinds of marine positioning systems, alone or combined with other marine systems, or integrated in different Earth-Sea Observatories, where the localization of the sensors is an issue.

Observations of Low Frequency Solar Radio Bursts from the Rosse Solar-Terrestrial Observatory

astro-ph — Thu, 05 Apr 2012 00:45:29 +0000

The Rosse Solar-Terrestrial Observatory (RSTO; www.rosseobservatory.ie) was established at Birr Castle, Co. Offaly, Ireland (53{\deg}05′38.9″, 7{\deg}55′12.7″) in 2010 to study solar radio bursts and the response of the Earth’s ionosphere and geomagnetic field. To date, three Compound Astronomical Low-cost Low-frequency Instrument for Spectroscopy and Transportable Observatory (CAL- LISTO) spectrometers have been installed, with the capability of observing in the frequency range 10-870 MHz. The receivers are fed simultaneously by biconical and log-periodic antennas. Nominally, frequency spectra in the range 10-400 MHz are obtained with 4 sweeps per second over 600 channels. Here, we describe the RSTO solar radio spectrometer set-up, and present dynamic spectra of a sample of Type II, III and IV radio bursts. In particular, we describe fine-scale structure observed in Type II bursts, including band splitting and rapidly varying herringbone features.

A Laser Frequency Comb System for Absolute Calibration of the VTT Echelle Spectrograph

astro-ph — Thu, 05 Apr 2012 00:44:04 +0000

A wavelength calibration system based on a laser frequency comb (LFC) was developed in a co-operation between the Kiepenheuer-Institut f\”ur Sonnenphysik, Freiburg, Germany and the Max-Planck-Institut f\”ur Quantenoptik, Garching, Germany for permanent installation at the German Vacuum Tower Telescope (VTT) on Tenerife, Canary Islands. The system was installed successfully in October 2011. By simultaneously recording the spectra from the Sun and the LFC, for each exposure a calibration curve can be derived from the known frequencies of the comb modes that is suitable for absolute calibration at the meters per second level. We briefly summarize some topics in solar physics that benefit from absolute spectroscopy and point out the advantages of LFC compared to traditional calibration techniques. We also sketch the basic setup of the VTT calibration system and its integration with the existing echelle spectrograph.