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Astron. Astrophys. 327, 513-521 (1997)

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The high-frequency compact radio structure
of the peculiar quasar 4C 39.25

A. Alberdi 1, 2, T.P. Krichbaum 3, D.A. Graham 3, A. Greve 4, M. Grewing 4, J.M. Marcaide 5, A. Witzel 3, R.S. Booth 6, L.B. Bååth 6, F. Colomer 6, 7, S. Doeleman 8, A.P. Marscher 9, A.E.E. Rogers 8, C.J. Schalinski 4, 10 and K. Standke 3

1 Laboratorio de Astrofsica Espacial y Fsica Fundamental, Apdo. 50727, E-28080 Madrid, Spain
2 Instituto de Astrofsica de Andaluca, CSIC, Apdo. 3004, Granada, Spain
3 Max Planck Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
4 IRAM, Domaine Universitaire, Grenoble, France
5 Departamento de Astronoma, Universitat de València, E-46100 Burjassot, Spain
6 Onsala Space Observatory, Onsala, Sweden
7 Observatorio de Yebes, Apdo. 148, Guadalajara, Spain
8 NEROC, Haystack Observatory, Westford, Massachusetts, USA
9 Boston University, Boston, MA, USA
10 now at Institut für Weltraumsensorik (DLR), Rudower Chaussee 5, D-12489 Berlin, Germany

Received 24 February 1997 / Accepted 2 April 1997


We present new high angular resolution images of the compact non-thermal radio source 4C 39.25 obtained from VLBI observations at [FORMULA] 1.3 cm, [FORMULA] 7 mm, and [FORMULA] 3 mm wavelengths. These maps and Gaussian modelfits show that the milli-arcsecond to sub-milliarcsecond structure of 4C 39.25 consists of a complex bent core-jet structure with embedded moving and stationary VLBI components. Facilitated by the small observing beams and high angular resolutions obtained at mm-wavelengths, we measured the relative positions of the jet components with an accuracy of a few hundred micro-arcseconds. This allows the detailed followup of the ongoing merging process of a westward superluminally moving component (b) with a stationary component a, located at [FORMULA] 2.9 mas east of the putative core d . In contrast to the other components of the structure with steeper spectra, the westernmost component d exhibits an inverted spectrum peaking between [FORMULA] 7 mm and [FORMULA] 3 mm, thus further supporting its identification as the VLBI core, self-absorbed at longer wavelengths.

From two VLBI maps obtained nearly simultaneously at [FORMULA] 7 mm and [FORMULA] 1.3 cm, we made the first spectral index map of 4C 39.25 in this wavelength regime. The main characteristics of the spectral index distribution of the jet are pronounced changes of the spectral index between orientations parallel and transverse to the jet axis. Near the merging components a and b the spectral index steepens with increasing separation from d . However, in the bridge of emission c, which connects d with a and b, the spectral index gradient has a direction transverse to the jet axis, suggesting a frequency dependent jet curvature and edge-brightening. A brief discussion of this behaviour within current jet models is presented.

Key words: galaxies: jets – quasars: 4C 39.25 – techniques: interferometric – radio continuum: general

Send offprint requests to: A. Alberdi

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998