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S.G. Marchenko, A.P. Marscher, J.R. Mattox, J. Hallum (IAA, Boston University), A.E. Wehrle (IPAC), S.D. Bloom (Hampden-Sydney College)
We have completed a program of monitoring of the milliarcesond- scale structure of \gamma-ray bright blazars with the VLBA at 22 and 43 GHz during the period from November 1993 to July 1997. The sample consists of 30 quasars and 12 BL Lac objects (only a subset of which was observed at each epoch). We have determined velocities of jet components in 33 sources, 20 of which have multiple moving knots. In 10 out of 20 superluminal components with sufficient \gamma-ray data, the epochs of zero separation coincide with times of global or local maxima of the \gamma-ray light curves within the 1-\sigma uncertainties of the epochs of zero separation. If we focus only on the \gamma-ray peaks, nine out of ten with sufficient VLBA data were accompanied by superluminal ejections. The \gamma-ray flares are therefore associated with energetic events in the jets rather than with minor episodes of enhanced particle acceleration.
We also report some of the statistical results from our VLBA study. The apparent velocities of the EGRET blazars are high, which agrees with the expectations of inverse Compton models in which the \gamma rays are more highly beamed than is the radio synchrotron radiation. We find evidence that the apparent speeds are higher farther from the core, which can be explained best by bending from a viewing angle near zero to one closer to \sin-1\,(\Gamma-1), rather than by increasing Lorentz factors. In 25 of the sources in the sample, there is at least one non-core component that appears to be stationary during our observations. We offer explanations for these results.
This research was funded by the NASA Gamma Ray Observatory Guest Investigator Program under grants NAG5-2508, NAG5-3829, and NAG5-7323.