Letter to the Editor

Dual-frequency VLBI study of Centaurus A on sub-parsec scales

The highest-resolution view of an extragalactic jet

¹ Dr. Remeis Sternwarte & ECAP, Universität Erlangen-Nürnberg, Sternwartstrasse 7, 96049 Bamberg, Germany
e-mail: cornelia.mueller@sternwarte.uni-erlangen.de
² Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany
³ CRESST/NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁴ Universities Space Research Association, 10211 Wincopin Circle, Suite 500 Columbia, MD 21044, USA
⁵ NASA, Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁶ CSIRO Astronomy and Space Science, Australia Telescope National Facility, PO Box 76, Epping, NSW 1710, Australia
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁸ Federal Agency for Cartography and Geodesy, Geodetic Observatory Wettzell, Sackenrieder Str. 25, 93444 Bad Kötzting, Germany
⁹ CSIRO Astronomy and Space Science, CDSCC, PO Box 1035, Tuggeranong, ACT 2901, Australia
¹⁰ Erlangen Centre for Astroparticle Physics, Erwin-Rommel Str. 1, 91058 Erlangen, Germany
¹¹ School of Mathematics & Physics, Private Bag 37, University of Tasmania, Hobart TAS 7001, Australia
¹² Nordic Optical Telescope, Apdo. de Correos 474, 38700 Santa Cruz de la Palma, Spain
¹³ Dept. d’Astronomia i Astrofísica, Universitat de València, 46100 Burjassot, València, Spain
¹⁴ Center for Astrophysics and Space Sciences, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA
¹⁵ Department of Physics and Astronomy, University of New Mexico, Albuquerque NM, 87131, USA
¹⁶ Curtin Institute of Radio Astronomy, Curtin University of Technology, Bentley, WA, 6102, Australia

Received: 28 January 2011
Accepted: 4 April 2011

Abstract

Context. Centaurus A is the closest active galactic nucleus. High resolution imaging using Very Long Baseline Interferometry (VLBI) enables us to study the spectral and kinematic behavior of the radio jet–counterjet system on sub-parsec scales, providing essential information for jet emission and formation models.

Aims. Our aim is to study the structure and spectral shape of the emission from the central-parsec region of Cen A.

Methods. As a target of the Southern Hemisphere VLBI monitoring program TANAMI (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry), VLBI observations of Cen A are made regularly at 8.4 and 22.3 GHz with the Australian Long Baseline Array (LBA) and associated telescopes in Antarctica, Chile, and South Africa.

Results. The first dual-frequency images of this source are presented along with the resulting spectral index map. An angular resolution of 0.4 mas  ×  0.7 mas is achieved at 8.4 GHz, corresponding to a linear scale of less than 0.013 pc. Hence, we obtain the highest resolution VLBI image of Cen A, comparable to previous space-VLBI observations. By combining with the 22.3 GHz image, which has been taken without contributing transoceanic baselines at somewhat lower resolution, we present the corresponding dual-frequency spectral index distribution along the sub-parsec scale jet revealing the putative emission regions for recently detected γ-rays from the core region by Fermi/LAT.

Conclusions. We resolve the innermost structure of the milliarcsecond scale jet and counterjet system of Cen A into discrete components. The simultaneous observations at two frequencies provide the highest resolved spectral index map of an AGN jet allowing us to identify multiple possible sites as the origin of the high energy emission.