Redshifted Fe Kα line from the unusual γ-ray source PMN J1603–4904

¹ Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany
e-mail: cornelia.mueller@astro.uni-wuerzburg.de
² Dr. Remeis Sternwarte & ECAP, Universität Erlangen-Nürnberg, Sternwartstrasse 7, 96049 Bamberg, Germany
³ Center for Space Science and Technology, University of Maryland Baltimore County, Baltimore, MD 21250, USA
⁴ CRESST and NASA, Goddard Space Flight Center, Astrophysics Science Division, Code 661, Greenbelt, MD 20771, USA
⁵ Catholic University of America, Washington, DC 20064, USA
⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ University of California, San Diego, Center for Astrophysics and Space Sciences, 9500 Gilman Drive, La Jolla, CA 92093-0424, USA
⁸ Institute of Space and Astronautical Science JAXA, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, 252-5210 Kanagawa, Japan
⁹ Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
¹⁰ Department of Physics and Astronomy, University of New Mexico, Albuquerque, NM 87131, USA

Received: 1 December 2014
Accepted: 23 December 2014

Abstract

Context. Multiwavelength observations have revealed the highly unusual properties of the γ-ray source PMN J1603–4904, which are difficult to reconcile with any other well-established γ-ray source class. The object is either a very atypical blazar or a compact jet source seen at a larger angle to the line of sight.

Aims. To determine the physical origin of the high-energy emission processes in PMN J1603–4904, we study the X-ray spectrum in detail.

Methods. We performed quasi-simultaneous X-ray observations with XMM-Newton and Suzaku in 2013 September. This resulted in the first high signal-to-noise X-ray spectrum of this source.

Results. The 2–10 keV X-ray spectrum can be well described by an absorbed power law with an emission line at 5.44 ± 0.05 keV (observed frame). We interpret this feature as a Kα line from neutral iron and accordingly determine the redshift of PMN J1603–4904 to be z = 0.18 ± 0.01, which corresponds to a luminosity distance of 872 ± 54 Mpc.

Conclusions. The detection of a redshifted X-ray emission line further challenges the original BL Lac classification of PMN J1603–4904. This result suggests that the source is observed at a larger angle to the line of sight than expected for blazars, and thus the source would add to the elusive class of γ-ray loud misaligned-jet objects, possibly a γ-ray bright young radio galaxy.