VLBI observations of CygnusA with sub-milliarcsecond resolution

Details VLBI observations of CygnusA with sub-milliarcsecond resolution VLBI observations of CygnusA with sub-milliarcsecond resolution

Jan 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998a%26a...329..873k&link_type=abstract

Astronomy and Astrophysics, v.329, p.873-894 (1998)

Astronomy and Astrophysics

Astrophysics

49

Galaxies: Active, Cygnus A, Jets, Radio Continuum: Galaxies, Techniques: Interferometric

Scientific paper

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^{\circ}. 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 (theta > 80^{\circ}, for R <2) with jet velocities in the range 0.2 h <= beta < 1 (H_0= 100 * h(-1) km sec(-1) Mpc(-1) ).

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