Multiepoch Very Long Baseline Array observations of egret-detected quasarsand BL lacertae objects: Superluminal motion of gamma-ray bright blazars

We present the results of a program to monitor the structure of the radio e mission in 42 gamma -ray bright blazars (31 quasars and 11 BL Lac objects) with the Very Long Baseline Array (VLBA) at 43, 22, and occasionally 15 and 8.4 GHz, over the period from 1993 November to 1997 July. We determine pro per motions in 33 sources and find that the apparent superluminal motions i n gamma -ray sources are much faster than for the general population of bri ght compact radio sources. This follows the strong dependence of the gamma -ray flux on the level of relativistic beaming for both external radiation Compton and synchrotron self-Compton emission. There is a positive correlat ion (correlation coefficient r = 0.45) between the flux density of the VLBI core and the gamma -ray flux and a moderate correlation (partial correlati on coefficient r = 0.31) between gamma -ray apparent luminosity and superlu minal velocities of jet components, as expected if the gamma -ray emission originates in a very compact region of the relativistic jet and is highly b eamed. In 43% of the sources the jet bends by more than 20 degrees on parse c scales, which is consistent with amplification by projection effects of m odest actual changes in position angle. In 27 of the sources in the sample there is at least one non-core component that appears to be stationary duri ng our observations. Different characteristics of stationary features close to and farther from the core lead us to suggest two different classes of s tationary components: those within about 2 mas of the core, probably associ ated with standing hydrodynamical compressions, and those farther down the jet, which tend to be associated with bends in the jet.