Parsec-scale jet behavior of NRAO 190 after a gamma-ray outburst in August1994

We present the results of a three-year Very Long Baseline Array (VLBA) imag ing campaign on the bright blazar NRAO 190 after it was identified as a sou rce of a prominent gamma-ray flare detected in August 1994 by the EGRET ins trument on the Compton Gamma Ray Observatory (McGlynn et al. 1997). The sou rce was observed at 22 GHz at 5 epochs and at 43 GHz once. As is typical fo r blazars, our results show a one-sided jet structure dominated by a bright , unresolved core with a prominent but gradually weakening knot moving down the jet with an apparent velocity of 8.5h(-1)c, (H-0 = 100h km s(-1) Mpc(- 1), q(0) = 0.5). The time of ejection of this knot can be extrapolated, to within the errors, to the epoch of the aforementioned gamma-ray flare. We i nterpret our results in terms of the "standard" relativistic shocked jet mo del (Blandford & Konigl 1979; Marscher & Gear 1985). The angular resolution of the VLBA, together with the results of Metsahovi 22 GHz total flux moni toring, allow us to examine the brightness variations of separate source co mponents. During the time of our monitoring, we observed the evolution of t he source shortly after the peak of a major outburst and also during and af ter a lower amplitude flare. The first event created a strong, superluminal ly moving component, while the latter produced no significant changes in th e jet structure. The characteristics of the jet were therefore different in the aftermath of each flare, possibly because the first disturbance genera ted rarefactions in its wake.