2000A&A...358..428Y

We present the results of a three-year Very Long Baseline Array (VLBA) imaging campaign on the bright blazar NRAO 190 after it was identified as a source of a prominent γ-ray flare detected in August 1994 by the EGRET instrument on the Compton Gamma Ray Observatory (McGlynn et al., 1997ApJ...481..625M). The source was observed at 22GHz at 5 epochs and at 43GHz once. As is typical for 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^–1c, (H₀=100h.km/s/Mpc, q₀=0.5). The time of ejection of this knot can be extrapolated, to within the errors, to the epoch of the aforementioned γ-ray flare. We interpret our results in terms of the ``standard'' relativistic shocked jet model (Blandford & Koenigl, 1979ApJ...232...34B; Marscher & Gear, 1985ApJ...298..114M). The angular resolution of the VLBA, together with the results of Metsaehovi 22 GHz total flux monitoring, allow us to examine the brightness variations of separate source components. During the time of our monitoring, we observed the evolution of the source shortly after the peak of a major outburst and also during and after a lower amplitude flare. The first event created a strong, superluminally moving component, while the latter produced no significant changes in the jet structure. The characteristics of the jet were therefore different in the aftermath of each flare, possibly because the first disturbance generated rarefactions in its wake.