Proper motion in Cygnus A

Abstract

Our recent VLBI observations of the prominent FR II radio galaxy Cygnus A with the EVN and the VLBA reveal a pronounced two-sided jet structure. At 5 GHz, we now have 4 epochs from 1986, 1991 (Carilli et al., 1991 & 1994), 1996 and 2002 from which we could derive the kinematics of the jet and counter-jet. On the jet side and on mas scales, the jet seems to accelerate from $β_{\rm app}\approx 0.1-0.2$ (Krichbaum et al. 1998) at core-separations near 1 mas to $β_{\rm app}\approx 0.4-0.6$ at $r \geq 4$ mas ($H_0=100$ km s$^{-1}$ Mpc$^{-1}$, $q_0=0.5$). For the first time we also measure significant structural variability on the counter-jet side. For this, we derive a motion of $β_{\rm app}=0.35\pm0.2$ at $r=9.5$ mas. The flat spectrum of the inner region of the counter-jet (free-free absorption) and the frequency dependence of the jet to counter-jet ratio support strong evidence for an obscuring torus in front of the counter-jet (Bach et al. in prep, Krichbaum et al. 1998).