Relativistic bulk motion in active galactic nuclei

Abstract

We discuss the evidence for relativistic bulk motion of the emitting plasma in the nuclei of ~100 radio sources, which include BL Lacertae objects, radio quasars, and radio galaxies, with published VBLI measurements of the core angular dimension and radio flux. Comparing the predicted and observed high-frequency (X-ray) flux, in the framework of the synchrotron self-Compton model, we derive the beaming or Doppler factor for all sources. This is compared with other beaming indicators, such as the value of the expansion velocity (mostly superluminal and available for ~40% of the objects) and the ratio of the core to the extended radio flux (available for all but two sources). We find that the different beaming indicators agree reasonably well, supporting the simple kinematic model of ballistic motion of knots in a jet. Furthermore, the mean values of the bulk Lorentz factor and the angle to the line of sight derived for the superluminal sources are consistent with those found from the attempts to unify BL Lacertae objects and radio-loud quasars with Fanaroff-Riley Types I and II radio galaxies, respectively.