Volume 562, February 2014
|Number of page(s)||11|
|Published online||30 January 2014|
The unusual multiwavelength properties of the gamma-ray source PMN J1603−4904
1 Dr. Remeis Sternwarte & ECAP, Universität Erlangen-Nürnberg, Sternwartstrasse 7, 96049 Bamberg, Germany
2 Institut für Theoretische Physik und Astrophysik, Universität Würzburg, am Hubland, 97074 Würzburg, Germany
3 NASA, Goddard Space Flight Center, Greenbelt MD 20771, USA
4 Catholic University of America, Washington DC 20064, USA
5 Max-Planck-Institut für Radioastronomie, auf dem Hügel 69, 53121 Bonn, Germany
6 Department of Physics and Astronomy, University of New Mexico, Albuquerque NM 87131, USA
7 Departamento de Astronomía, Universidad de Concepción, 160 C Casilla, Concepción, Chile
8 CSIRO Astronomy and Space Science, ATNF, PO Box 76 Epping, Sydney NSW 1710, Australia
9 Bundesamt für Kartographie und Geodäsie, 93444 Bad Kötzting, Germany
10 CSIRO Astronomy and Space Science, Canberra Deep Space Communications Complex, PO Box 1035, ACT 2901 Tuggeranong, Australia
11 School of Mathematics & Physics, University of Tasmania, Private Bag 37, Hobart 7001 TAS, Australia
12 Nordic Optical Telescope, Apartado 474, E 38700 Santa Cruz de La Palma, Spain
13 Hartebeesthoek Radio Astronomy Observatory, 1740 Krugersdorp, South Africa
14 Observatori Astronòmic, Universitat de València, Parc Científic, C. Catedrático José Beltrán 2, 46980 Paterna, València, Spain
15 Departament d’Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, 46100 Burjassot, València, Spain
16 International Centre for Radio Astronomy Research, Curtin University, 6102 Perth, Australia
Received: 10 October 2013
Accepted: 12 December 2013
Context. We investigate the nature and classification of PMN J1603−4904, a bright radio source close to the Galactic plane, which is associated with one of the brightest hard-spectrum γ-ray sources detected by Fermi/LAT. It has previously been classified as a low-peaked BL Lac object based on its broadband emission and the absence of optical emission lines. Optical measurements, however, suffer strongly from extinction and the absence of pronounced short-time γ-ray variability over years of monitoring is unusual for a blazar.
Aims. In this paper, we are combining new and archival multiwavelength data of PMN J1603−4904 in order to reconsider the classification and nature of this unusual γ-ray source.
Methods. For the first time, we study the radio morphology of PMN J1603−4904 at 8.4 GHz and 22.3 GHz, and its spectral properties on milliarcsecond scales, based on VLBI observations from the TANAMI program. We combine the resulting images with multiwavelength data in the radio, IR, optical/UV, X-ray, and γ-ray regimes.
Results. PMN J1603−4904 shows a symmetric brightness distribution at 8.4 GHz on milliarcsecond scales, with the brightest, and most compact component in the center of the emission region. The morphology is reminiscent of a compact symmetric object (CSO). Such objects, thought to be young radio galaxies, have been predicted to produce γ-ray emission but have not been detected as a class by the Fermi γ-ray telescope so far. Sparse (u,v)-coverage at 22.3 GHz prevents an unambiguous modeling of the source morphology at this higher frequency. Moreover, infrared measurements reveal an excess in the spectral energy distribution (SED), which can be modeled with a blackbody with a temperature of about 1600 K, and which is usually not present in blazar SEDs.
Conclusions. The TANAMI VLBI data and the shape of the broadband SED challenge the current blazar classification of one of the brightest γ-ray sources in the sky. PMN J1603−4904 seems to be either a highly peculiar BL Lac object or a misaligned jet source. In the latter case, the intriguing VLBI structure opens room for a possible classification of PMN J1603−4904 as a γ-ray bright CSO.
Key words: galaxies: active / galaxies: individual: PMN J1603-4904 / radio continuum: general / gamma rays: galaxies / techniques: interferometric / X-rays: galaxies
© ESO, 2014
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