Multiepoch Very Long Baseline Array Observations of EGRET‐detected Quasars and BL Lacertae Objects: Superluminal Motion of Gamma‐Ray Bright Blazars

We present the results of a program to monitor the structure of the radioemission in 42 $\gamma$-ray bright blazars (31 quasars and 11 BL Lac objects)with the VLBA at 43, 22, and occasionally 15 and 8.4 GHz, over the period fromNovember 1993 to July 1997. We determine proper motions in 33 sources and findthat the apparent superluminal motions in $\gamma$-ray sources are much fasterthan for the general population of bright compact radio sources. This followsthe strong dependence of the $\gamma$-ray flux on the level of relativisticbeaming for both external-radiation Compton and synchrotron self-Comptonemission. There is a positive correlation (correlation coefficient $r$=0.45)between the flux density of the VLBI core and the $\gamma$-ray flux and amoderate correlation (partial correlation coefficient $r$=0.31) between$\gamma$-ray apparent luminosity and superluminal velocities of jet components,as expected if the $\gamma$-ray emission originates in a very compact region ofthe relativistic jet and is highly beamed. In 43% of the sources the jet bendsby more than 20$^\circ$ on parsec scales, which is consistent withamplification by projection effects of modest actual changes in position angle. In 27 of the sources in the sample there is at least one non-core componentthat appears to be stationary during our observations. Differentcharacteristics of stationary features close to and farther from the core leadus to suggest two different classes of stationary components: those withinabout 2 milliarcseconds (mas) of the core, probably associated with standinghydrodynamical compressions, and those farther down the jet, which tend to beassociated with bends in the jet.Comment: 162 pages in ApJ (preprint) format, 9 tables, 48 figures, some of which have been bitmapped, accepted for publication in ApJS (v. 134