Discovery of an Ultrasoft X-ray Transient Source in the 2XMM Catalog: a Tidal Disruption Event Candidate
(2 votes over all institutions)
We have discovered an ultrasoft X-ray transient source, 2XMMi J184725.1-631724, which was detected serendipitously in two {\it XMM-Newton} observations in the direction of the center of the galaxy IC 4765-f01-1504 at a redshift of 0.0353. These two observations were separated by 211 days, with the 0.2–10 keV absorbed flux increasing by a factor of about 9. Their spectra are best described by a model dominated by a thermal disk or a single-temperature blackbody component (contributing $\gtrsim$80% of the flux) plus a weak power-law component. The thermal emission has a temperature of a few tens of eV, and the weak power-law component has a photon index of $\sim$3.5. Similar to the black hole X-ray binaries in the thermal state, our source exhibits an accretion disk whose luminosity appears to follow the $L\propto T^4$ relation. This would indicate that the black hole mass is about $10^5$–$10^6$ {$M_{\odot}$} using the best-fitting inner disk radius. Both {\it XMM-Newton} observations show variability of about 21% on timescales of hours, which can be explained as due to fast variations in the mass accretion rate. The source was not detected by {\it ROSAT} in an observation in 1992, indicating a variability factor of $\gtrsim$64 over longer timescales. The source was not detected again in X-rays in a {\it Swift} observation in 2011 February, implying a flux decrease by a factor of $\gtrsim$12 since the last {\it XMM-Newton} observation. The transient nature, in addition to the extreme softness of the X-ray spectra and the inactivity of the galaxy implied by the lack of strong optical emission lines, makes it a candidate tidal disruption event. If this is the case, the first {\it XMM-Newton} observation would have been in the rising phase, and the second one in the decay phase.
