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

I. The X-ray View

¹ Lehrstuhl für Astronomie, Universität Würzburg, Campus Hubland Nord, Emil-Fischer-Straße 31, 97074 Würzburg, Germany
e-mail: akreikenbohm@astro.uni-wuerzburg.de
² Dr Karl Remeis-Observatory and Erlangen Centre for Astroparticle Physics, Sternwartstr. 7, 96049 Bamberg, Germany
³ University of California, San Diego, CASS, 9500 Gilman Dr., MC 0424, La Jolla, CA 92093-0424, USA
⁴ Alexander von Humboldt Fellow  
⁵ Joint ALMA Observatory, ESO, 763 0355 Santiago, Chile
⁶ Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁷ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmengen, The Netherlands
⁸ Catholic University of America, Washington, DC 20064, USA
⁹ University of Maryland Baltimore County/CRESST, Baltimore, MD 21250, USA
¹⁰ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹¹ Observatori Astronòmic, Univ. València, 46980 Paterna València, Spain
¹² Dept. d’Astronomia i Astrofísica, Univ. València, 46100 Burjassot, València, Spain

Received: 15 August 2014
Accepted: 2 October 2015

Abstract

As part of the TANAMI multiwavelength progam, we discuss new X-ray observations of the γ-ray and radio-loud narrow line Seyfert 1 galaxy (γ-NLS1) PKS 2004−447. The active galaxy is a member of a small sample of radio-loud NLS1s detected in γ-rays by the Fermi Large Area Telescope. It stands out for being the radio-loudest and the only southern-hemisphere source in this sample. We present results from our X-ray monitoring program comprised of Swift snapshot observations from 2012 through 2014 and two new X-ray observations with XMM-Newton in 2012. Supplemented by archival data from 2004 and 2011, our data set allows for a careful analysis of the X-ray spectrum and variability of this peculiar source. The (0.5–10) keV spectrum is described well by a power law (Γ ~ 1.6), which can be interpreted as non-thermal emission from a relativistic jet. The source exhibits moderate flux variability on timescales of both months and years. Correlated brightness variations in the (0.5–2) keV and (2–10) keV bands are explained by a single variable spectral component, such as the one from the jet. A possible soft excess seen in the data from 2004 cannot be confirmed by the new XMM-Newton observations taken during low-flux states. Any contribution to the total flux in 2004 is less than 20% of the power-law component. The (0.5–10) keV luminosities of PKS 2004−447 are in the range of (0.5−2.7) × 10⁴⁴ erg s^-1. A comparison of the X-ray properties among the known γ-NLS1 galaxies shows that in four out of five cases the X-ray spectrum is dominated by a flat power law without intrinsic absorption. These objects are moderately variable in their brightness, while spectral variability is observed in at least two sources. The major difference across the X-ray spectra of γ-NLS1s is the luminosity, which spans a range of almost two orders of magnitude from 10⁴⁴ erg s^-1 to 10⁴⁶ erg s^-1 in the (0.5–10) keV band.