The Parsec-Scale Structure and Evolution of PKS 0521−365

Abstract

PKS 0521-365 is a relatively nearby active galactic nucleus possessing a bright compact radio source, strong extended radio and X-ray emission, broad and variable nuclear optical emission lines, and one of the best examples of an optical synchrotron jet. Modeling of the high-energy emission from this source, ratios of core to extended luminosity at radio and optical wavelengths, and the morphology of the extended radio and optical jet indicate that the emission from the nucleus of PKS 0521-365 is not likely to be strongly affected by relativistic boosting. We present multiepoch and multifrequency VLBI data for PKS 0521-365 that show that any apparent motion of components in the jet must be less than 1.2c, that the jet to counterjet surface brightness ratio on the parsec-scale is greater than 20, and that the peak brightness temperature of the source is greater than 1 × 1011 K at 5 GHz. While these results are consistent with the idea that PKS 0521-365 is not highly beamed, the combination of the apparent motion and jet-to-counterjet limits with Doppler-factor lower limits from synchrotron self-Compton and gamma-ray transparency models do not tightly constrain the jet speed or orientation to values which compel the conclusion that PKS 0521-365 is weakly beamed. The data are formally consistent with weak beaming as well as strong beaming. However, calculations by Pian et al. based on inhomogeneous jet models for the spectral energy distribution indicate that the Doppler factor is close to unity and place limits on the allowed ranges of jet speed and angle to the line of sight. These calculations are consistent with the VLBI data, although future higher sensitivity VLBI observations have the potential to test the results of these models more strictly. Although we find no significant support for superluminal motion in PKS 0521-365, we do find evidence for evolution internal to a component in the parsec-scale jet, similar to that seen on the subparsec-scale in Centaurus A and M87.