1998A&A...329..873K

We present new images of the two-sided VLBI-structure of the radio galaxy CygnusA at 1.6GHz, 22GHz and 43GHz. The maps show the jet and counter-jet of CygnusA on scales from ∼400 to 0.1 milliarcseconds, corresponding to projected sizes of ∼300pc to 0.07pc. Our best 22GHz image shows a small (few degree) misalignment of the VLBI-jets with the kpc-jets, longitudinal oscillations of the jet's ridge line with amplitudes of ≤0.6mas, and transverse oscillations of the jet's width with amplitudes of 0.2-0.3mas. At distances of <15mas from the core, both jets expand at an opening angle of ∼5°. Between 22 and 43GHz the core has an inverted spectrum. The spectrum of the jet appears steep, that of the counter-jet relatively flat. From two 22GHz observations we find evidence for apparent acceleration (from 0.1c to 0.7c) along the jet. The acceleration could be jet-intrinsic or related to phase velocities. In the counter-jet structural variations seem also to be present, but cannot accurately be determined. Between 1.6GHz and 43GHz the jet-to-counter-jet ratio R is frequency dependent with a maximum of R≃5 near 5GHz and a smaller ratio of R =1-2 at 1.6GHz and 43GHz. This can be interpreted as an effect of absorption by a partially opaque inclined absorber (e.g. a disk or a torus) obscuring the counter-jet but not the jet. With an absorption corrected intrinsically small jet-to-counter-jet ratio, CygnusA could be oriented at a relatively large angle to the line of sight (θ> 80°, for R<2) with jet velocities in the range 0.2h≤β<1 (H₀=100xh^–1km/s/Mpc).