VLBI observations of CygnusA with sub-milliarcsecond resolution

We present new images of the two-sided VLBIstructure 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 ∼ 300 pc to 0.07 pc. 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 0.1 c to 0.7 c) 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-counterjet 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 = 100 · h−1km sec−1Mpc−1).