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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 Ξ²appβ‰ˆ0.1βˆ’0.2\beta_{\rm app}\approx 0.1-0.2 (Krichbaum et al. 1998) at core-separations near 1 mas to Ξ²appβ‰ˆ0.4βˆ’0.6\beta_{\rm app}\approx 0.4-0.6 at rβ‰₯4r \geq 4 mas (H0=100H_0=100 km sβˆ’1^{-1} Mpcβˆ’1^{-1}, q0=0.5q_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 Ξ²app=0.35Β±0.2\beta_{\rm app}=0.35\pm0.2 at r=9.5r=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).Comment: 4 pages, 6 figures, appears in: Proceedings of the 6th European VLBI Network Symposium held on June 25th-28th in Bonn, Germany. Edited by: E. Ros, R.W. Porcas, A.P. Lobanov, and J.A. Zensu

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