Multi-frequency & Multi-epoch VLBI study of Cygnus A
Abstract
We present the first multi-frequency phase-referenced observations of Cygnus A done with the VLBA at 15 and 22 GHz. We find a pronounced two-sided jet structure, with a steep spectrum along the jet and a highly inverted spectrum towards the counter-jet. The inverted spectrum and the frequency dependent jet to counter-jet ratio suggest an obscuring torus in front of the counter-jet. From 14 epochs of 15 GHz VLBA data we accurately derive the jet and counter-jet kinematics. For the inner jet (r <= 5 mas) we measure motions of betaapp ~ 0.2-0.5 h-1 and on the counter-jet side we find beta app ~ 0.03+-0.02 h-1. We discuss the jet velocities within the unified jet model.
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