Deconstructing blazars: A different scheme for jet kinematics in
  flat-spectrum AGN
  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. We develop a number of tools that extract information about
the apparent linear and angular evolution of the CJF jet ridge lines, as well
as their morphology. In this way, we study both radial and non-radial apparent
motions in the CJF jets. We find that approximately half of the sample shows
appreciable apparent jet widths ($>10 degrees$), with BL Lac jet ridge lines
showing significantly larger apparent widths than 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 to 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 show "BL Lac like" behavior, compared to their
non-variable counterparts.
