Radio-to-γ-ray monitoring of the narrow-line Seyfert 1 galaxy PMN J0948 + 0022 from 2008 to 2011^⋆,⋆⋆

¹ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate ( LC), Italy
e-mail: luigi.foschini@brera.inaf.it
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Cahill Center for Astronomy & Astrophysics, Caltech, 1200 E. California Blvd, Pasadena, CA 91125, USA
⁴ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁵ Department of Physics, Purdue University, West Lafayette, IN 47907, USA
⁶ Department of Astronomy and Physics, Saint Mary’s University, 923 Robie Street, Halifax, NS B3H 3C3, Canada
⁷ Department of Natural Sciences, Shawnee State University, 940 2nd Street, Portsmouth, OH 45662, USA
⁸ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
⁹ Crimean Astrophysical Observatory, 98409 Nauchny, Crimea, Ukraine
¹⁰ Pulkovo Observatory, 196140 St. Petersburg, Russia
¹¹ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹² ITPA, Universität Würzburg, Campus Hubland Nord, Emil-Fischer-Str. 31, 97074 Würzburg, Germany
¹³ ISDC Data Centre for Astrophysics, Chemin d’Ecogia 16, 1290 Versoix, Switzerland
¹⁴ Instituto Nacional de Astrofísica, Óptica y Electrónica, Tonantzintla, 72840 Puebla, Mexico
¹⁵ Department of Astronomy & Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
¹⁶ INAF – Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
¹⁷ Astro Space Center of the Lebedev Physical Institute, Profsoyuznaya 84/32, 117997 Moscow, Russia
¹⁸ Instituto de Radioastronomía Milimétrica (IRAM), Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain
¹⁹ SISSA, via Bonomea 265, 34136 Trieste, Italy
²⁰ Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, 229-8510 Kanagawa, Japan

Received: 14 August 2012
Accepted: 24 September 2012

Abstract

We present more than three years of observations at different frequencies, from radio to high-energy γ-rays, of the Narrow-Line Seyfert 1 (NLS1) Galaxy PMN J0948 + 0022 (z = 0.585). This source is the first NLS1 detected at energies above 100 MeV and therefore can be considered the prototype of this emerging new class of γ-ray emitting active galactic nuclei (AGN). The observations performed from 2008 August 1 to 2011 December 31 confirmed that PMN J0948 + 0022 generates a powerful relativistic jet, which is able to develop an isotropic luminosity at γ-rays of the order of 10⁴⁸ erg s^-1, at the level ofpowerful quasars. The evolution of the radiation emission of this source in 2009 and 2010 followed the canonical expectations of relativistic jets with correlated multiwavelength variability (γ-rays followed by radio emission after a few months), but it was difficult to retrieve a similar pattern in the light curves of 2011. The comparison of γ-ray spectra before and including 2011 data suggested that there was a softening of the high-energy spectral slope. We selected five specific epochs to be studied by modelling the broad-band spectrum, which are characterised by an outburst at γ-rays or very low/high flux at other wavelengths. The observed variability can largely be explained by changes in the injected power, the bulk Lorentz factor of the jet, or the electron spectrum. The characteristic time scale of doubling/halving flux ranges from a few days to a few months, depending on the frequency and the sampling rate. The shortest doubling time scale at γ-rays is 2.3 ± 0.5 days. These small values underline the need of highly sampled multiwavelength campaigns to better understand the physics of these sources.