2012A&A...537A.112K

Recent VLBI studies of the morphology and kinematics of individual BL Lac objects (S5 1803+784, PKS 0735+178, etc.) have revealed a new paradigm for the pc-scale jet kinematics of these sources. Unlike the apparent superluminal outward motions usually observed in blazars, most, if not all, jet components in these sources appear to be stationary with respect to the core, while exhibiting strong changes in their position angles. As a result, the jet ridge lines of these sources evolve substantially, at times forming a wide channel-flow. We investigate the Caltech-Jodrell Bank flat-spectrum (CJF) sample of radio-loud active galaxies to study this new kinematic scenario for flat-spectrum AGN. Comparing BL Lac objects and quasars in the CJF, we look for differences in the kinematics and morphologies of their jet ridge lines. The large number of sources in the CJF sample, together with the excellent kinematical data available, allow us to perform a robust statistical analysis in this context. We develop a number of tools to extract information about the apparent linear and angular evolution of the CJF jet ridge lines, as well as their morphology. We study both the radial and non-radial apparent motions in the CJF jets. A statistical analysis of the extracted information allows us to test this new kinematic scenario and assess the importances of the radial and non-radial motions in flat-spectrum AGN jets in general and especially in BL Lacs compared to quasars. We also use these tools to check the kinematics for (multi-wavelength) variable AGN. We find that approximately half of the sample display appreciable apparent jet widths (>10 degrees), with BL Lac jet ridge lines showing significantly larger apparent widths than those of both quasars and radio galaxies. In addition, BL Lac jet ridge lines are found to change their apparent width more strongly. Finally, BL Lac jet ridge lines show the least apparent linear evolution, which translates into the smallest apparent expansion speeds for their components. We find compelling evidence supporting a substantially different kinematic scenario for flat-spectrum radio-AGN jets and in particular for BL Lac objects. In addition, we find that variability is closely related to the properties of a source's jet ridge line. Variable quasars are found to have ``BL Lac like'' behavior, relative to their non-variable counterparts.