The gamma-ray emitting radio-loud narrow-line Seyfert 1 galaxy PKS 2004−447

II. The radio view^⋆

¹ Lehrstuhl für Astronomie, Universität Würzburg, Campus Hubland Nord, Emil-Fischer-Str. 31, 97074 Würzburg, Germany
e-mail: robert.schulz@physik.uni-wuerzburg.de
² Dr. Remeis-Sternwarte & ECAP, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
³ NASA, Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
⁴ University of Maryland, Baltimore County, 1000 Hilltop Cir, Baltimore, MD 21250, USA
⁵ The Catholic University of America, 620 Michigan Ave NE, Washington, DC 20064, USA
⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ Observatori Astronòmic, Univ. València, 46980 Paterna València, Spain
⁸ Dept. Astronomia i Astrofìsica, Univ. València, 46100 Burjassot, València, Spain
⁹ CSIRO, Astronomy and Space Science, ATNF, PO Box 76 Epping, NSW 1710, Australia
¹⁰ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85741 Garching, Germany
¹¹ Bundesamt für Kartographie und Geodäsie, 93444 Bad Kötzingen, Germany
¹² CSIRO Astronomy and Space Science, Canberra Deep Space Communications Complex, PO Box 1035, Tuggeranong ACT 2901, Australia
¹³ School of Mathematics & Physics, University of Tasmania, Private Bag 37, Hobart, 7001 Tasmania, Australia
¹⁴ University of California, San Diego, CASS, 9500 Gilman Dr., MC 0424, La Jolla, CA 92093-0424, USA
¹⁵ Department of Astrophysics/MAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁶ Hartebeesthoek Radio Astronomy Observatory, 1740 Krugersdorp, South Africa

Received: 18 September 2015
Accepted: 25 October 2015

Abstract

Context. Γ-ray-detected radio-loud narrow-line Seyfert 1 (γ-NLS1) galaxies constitute a small but interesting sample of the γ-ray-loud AGN. The radio-loudest γ-NLS1 known, PKS 2004−447, is located in the southern hemisphere and is monitored in the radio regime by the multiwavelength monitoring programme TANAMI.

Aims. We aim for the first detailed study of the radio morphology and long-term radio spectral evolution of PKS 2004−447, which are essential for understanding the diversity of the radio properties of γ-NLS1s.

Methods. The TANAMI VLBI monitoring program uses the Australian Long Baseline Array (LBA) and telescopes in Antarctica, Chile, New Zealand, and South Africa to monitor the jets of radio-loud active galaxies in the southern hemisphere. Lower resolution radio flux density measurements at multiple radio frequencies over four years of observations were obtained with the Australia Telescope Compact Array (ATCA).

Results. The TANAMI VLBI image at 8.4 GHz shows an extended one-sided jet with a dominant compact VLBI core. Its brightness temperature is consistent with equipartition, but it is an order of magnitude below other γ-NLS1s with the sample value varying over two orders of magnitude. We find a compact morphology with a projected large-scale size < 11 kpc and a persistent steep radio spectrum with moderate flux-density variability.

Conclusions. PKS 2004−447 appears to be a unique member of the γ-NLS1 sample. It exhibits blazar-like features, such as a flat featureless X-ray spectrum and a core-dominated, one-sided parsec-scale jet with indications for relativistic beaming. However, the data also reveal properties atypical for blazars, such as a radio spectrum and large-scale size consistent with compact-steep-spectrum (CSS) objects, which are usually associated with young radio sources. These characteristics are unique among all γ-NLS1s and extremely rare among γ-ray-loud AGN.