VLBI OBSERVATIONS OF CYGNUS-A WITH SUBMILLIARCSECOND RESOLUTION

We present new images of the two-sided VLBI-structure of the radio gal axy Cygnus A at 1.6 GHz, 22 GHz and 43 GHz. The maps show the jet and counter-jet of Cygnus A on scales from similar to 400 to 0.1 milliarcs econds, corresponding to projected sizes of similar to 300 pc to 0.07 pc. Our best 22 GHz image shows a small (few degree) misalignment of t he VLBI-jets with the kpc-jets, longitudinal oscillations of the jet's ridge line with amplitudes of less than or equal to 0.6 mas, and tran sverse oscillations of the jet's width with amplitudes of 0.2 - 0.3 ma s. At distances of < 15 mas from the core, both jets expand at an open ing angle of similar to 5 degrees. Between 22 and 43 GHz the core has an inverted spectrum. The spectrum of the jet appears steep, that of t he counter-jet relatively flat. From two 22 GHz observations we find e vidence for apparent acceleration (from 0.1 c to 0.7 c) along the jet. The acceleration could be jet-intrinsic or related to phase velocitie s. In the counter-jet structural variations seem also to be present, b ut cannot accurately be determined. Between 1.6 GHz and 43 GHz the jet -to-counter-jet ratio R is frequency dependent with a maximum of R sim ilar or equal to 5 near 5 GHz and a smaller ratio of R = 1 - 2 at 1.6 GHz and 43 GHz. This can be interpreted as an effect of absorption by a partially opaque inclined absorber (e.g. a disk or a torus) obscurin g the counter-jet but not the jet. With an absorption corrected intrin sically small jet-to-counter-jet ratio, Cygnus A could be oriented at a relatively large angle to the line of sight (theta > 80 degrees, for R < 2) with jet velocities in the range 0.2 h less than or equal to b eta < 1 (H-o = 100 . h(-1) km sec(-1) Mpc(-1)).