Radio-optical scrutiny of compact AGN: Correlations between properties of pc-scale jets and optical nuclear emission
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We study the correlations between the VLBA (Very Long Baseline Array) radio emission at 15 GHz, extended emission at 151 MHz, and optical nuclear emission at 5100\,\AA\ for a complete sample of 135 compact jets. We use the partial Kendall’s tau correlation analysis to check the link between radio properties of parsec-scale jets and optical luminosities of host active galactic nuclei (AGN). We find a significant positive correlation for 99 quasars between optical nuclear luminosities and total radio (VLBA) luminosities of unresolved cores at 15\,GHz originated at milliarcseconds scales. For 18 BL Lacs, the optical continuum emission correlates with the radio emission of the jet at 15\,GHz. We suggest that the radio and optical emission are beamed and originate in the innermost part of the sub–parsec-scale jet in quasars. Analysis of the relation between the apparent speed of the jet and the optical nuclear luminosity at 5100\,\AA\ supports the relativistic beaming model for the optical emission generated in the jet, and allows the peak values of the intrinsic optical luminosity of the jet and its Lorentz factor to be estimated for the populations of quasars ($2\times10^{20}$W\,Hz$^{-1}$ and $\gamma=52$), BL Lacs ($9\times10^{21}$W\,Hz$^{-1}$ and $\gamma=20$), and radio galaxies ($1.5\times10^{21}$W\,Hz$^{-1}$ and $\gamma=9$). The radio-loudness of quasars (the ratio of 15\,GHz flux density and optical nuclear flux at 5100\,\AA) is found to increase at high redshifts, which can be a result of lower efficiency of the accretion in AGN having higher radio luminosities. A strong positive correlation is found between the intrinsic kinetic power of the jet (measured from the flux density at 151\,MHz) and the apparent luminosities of the total and the unresolved core emission of the jet at 15\,GHz. This correlation is interpreted in terms of intrinsically more luminous parsec-scale jet producing more luminous extended structure which is detectable at low radio frequencies, 151\,MHz. A possibility that the low frequency radio emission is relativistically beamed in superluminal AGN and therefore correlates with radio luminosity of the jet at 15\,GHz can not be ruled out. Monitoring of superluminal AGN in a wide range of frequencies is required to check the contribution of each effect.
