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Astron. Astrophys. 329, 873-894 (1998)


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VLBI observations of Cygnus A with sub-milliarcsecond resolution

T.P. Krichbaum 1, W. Alef 1, A. Witzel 1, J.A. Zensus 2, 1, R.S. Booth 3, A. Greve 4 and A.E.E. Rogers 5

1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
2 National Radio Astronomy Observatory, Charlottesville, VA 22903, USA
3 Onsala Space Observatory, Chalmers University of Technology, S-43992 Onsala, Sweden
4 Institut de Radioastronomie Millimétrique, F-38406 St. Martin d'Hères, France
5 MIT-NEROC Haystack Observatory, Westford, MA 01886, USA

Received 3 July 1997 / Accepted 5 September 1997

Abstract

We present new images of the two-sided VLBI-structure of the radio galaxy 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 [FORMULA] to 0.1 milliarcseconds, corresponding to projected sizes of [FORMULA] pc to 0.07 pc. Our best 22 GHz 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 [FORMULA] mas, and transverse oscillations of the jet's width with amplitudes of [FORMULA] mas. At distances of [FORMULA] mas from the core, both jets expand at an opening angle of [FORMULA] [FORMULA]. Between 22 and 43 GHz the core has an inverted spectrum. The spectrum of the jet appears steep, that of the counter-jet relatively flat. From two 22 GHz 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 1.6 GHz and 43 GHz the jet-to-counter-jet ratio R is frequency dependent with a maximum of R [FORMULA] near 5 GHz and a smaller ratio of R [FORMULA] 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) obscuring the counter-jet but not the jet. With an absorption corrected intrinsically small jet-to-counter-jet ratio, Cygnus A could be oriented at a relatively large angle to the line of sight ([FORMULA] [FORMULA], for R [FORMULA]) with jet velocities in the range [FORMULA] ([FORMULA]).

Key words: galaxies: active; Cygnus A; jets – radio continuum: galaxies – techniques: interferometric

Send offprint requests to: T.P. Krichbaum

© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997
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