Multi-frequency & Multi-epoch VLBI study of Cygnus A

Research paper by U. Bach, M. Kadler, T. P. Krichbaum, E. Middelberg, W. Alef, A. Witzel, J. A. Zensus

Indexed on: 15 Sep '03Published on: 15 Sep '03Published in: Astrophysics


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 beta_app ~ 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.