Two active states of the narrow-line gamma-ray-loud AGN GB 1310+487

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: kirx@scan.sai.msu.ru
² Astro Space Center of Lebedev Physical Institute, Profsoyuznaya Str. 84/32, 117997 Moscow, Russia
³ Sternberg Astronomical Institute, Moscow State University, Universitetskii pr. 13, 119992 Moscow, Russia
⁴ Department of Physics and Astronomy, University of New Mexico, Albuquerque NM, 87131, USA
⁵ Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1, Kagamiyama, Higashi-Hiroshima, 739-8526 Hiroshima, Japan
⁶ INAF/IASF–Bologna, via Gobetti 101, 40129 Bologna, Italy
⁷ Instituto Nacional de Astrofisica, Optica y Electronica, Tonantzintla, Puebla, CP 72860 Mexico, Mexico
⁸ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
⁹ NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁰ National Research Council Research Associate, National Academy of Sciences, Washington, DC 20001, resident at Naval Research Laboratory, Washington, DC 20375, USA
¹¹ Dip. di Fisica, Universitá degli Studi di Perugia, via A. Pascoli, 060123 Perugia, Italy
¹² INFN – Sezione di Perugia, via A. Pascoli, 06123 Perugia, Italy
¹³ INAF-IRA Bologna, Via Gobetti 101, 40129 Bologna, Italy
¹⁴ Université Bordeaux 1, CNRS/IN2p3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France
¹⁵ Laboratoire Leprince-Ringuet, École polytechnique, CNRS/IN2P3, 91120 Palaiseau, France
¹⁶ U.S. Naval Research Laboratory, Code 7653, 4555 Overlook Ave. SW, Washington, DC 20375-5352, USA
¹⁷ Department of Physical Sciences, Hiroshima University, Higashi- Hiroshima, Hiroshima 739-8526, Japan
¹⁸ Department of Physics, Stanford University, Stanford, CA 94305, USA
¹⁹ Department of Astronomy, Stockholm University, 106 91 Stockholm, Sweden
²⁰ The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, 106 91 Stockholm, Sweden
²¹ Department of Physics, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
²² Department of Physics, Purdue University, 525 Northwestern Ave, West Lafayette, IN 47907, USA
²³ Univ. Bordeaux, CENBG, UMR 5797, 33170 Gradignan, France
²⁴ CNRS, IN2P3, CENBG, UMR 5797, 33170 Gradignan, France
²⁵ Cahill Center for Astronomy andAstrophysics, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91101, USA
²⁶ ASI–ASDC, via G. Galilei, 00044 Frascati, Roma, Italy
²⁷ Nordic Optical Telescope, Apdo. de Correos 474, 38700 Santa Cruz de la Palma, Spain
²⁸ Pulkovo Observatory, Pulkovskoe Chaussee 65/1, 196140 St. Petersburg, Russia
²⁹ Crimean Astrophysical Observatory, 98409 Nauchny, Crimea, Ukraine
³⁰ Instituto Radioastronomía Milimétrica„ Avenida Divina Pastora 7, Local 20, 18012, Granada, Spain
³¹ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, 606-8502 Kyoto, Japan
³² INAF/IASF–Palermo, via U. La Malfa 153, 90146 Palermo, Italy
³³ Kwasan Observatory, Kyoto University, Ohmine-cho Kita Kazan, Yamashina-ku, 607-8471 Kyoto, Japan

Received: 7 November 2012
Accepted: 8 January 2014

Abstract

Context. Previously unremarkable, the extragalactic radio source GB 1310+487 showed a γ-ray flare on 2009 November 18, reaching a daily flux of ~ 10^-6 photons cm^-2 s^-1 at energies E> 100 MeV and became one of the brightest GeV sources for about two weeks. Its optical spectrum shows strong forbidden-line emission while lacking broad permitted lines, which is not typical for a blazar. Instead, the spectrum resembles those of narrow emission-line galaxies.

Aims. We investigate changes in the object’s radio-to-GeV spectral energy distribution (SED) during and after the prominent γ-ray flare with the aim of determining the nature of the object and of constraining the origin of the variable high-energy emission.

Methods. The data collected by the Fermi and AGILE satellites at γ-ray energies; Swift at X-ray and ultraviolet (UV); the Kanata, NOT, and Keck telescopes at optical; OAGH and WISE at infrared (IR); and IRAM 30 m, OVRO 40 m, Effelsberg 100 m, RATAN-600, and VLBA at radio are analyzed together to trace the SED evolution on timescales of months.

Results. The γ-ray/radio-loud narrow-line active galactic nucleus (AGN) is located at redshift z = 0.638. It shines through an unrelated foreground galaxy at z = 0.500. The AGN light is probably amplified by gravitational lensing. The AGN SED shows a two-humped structure typical of blazars and γ-ray-loud narrow-line Seyfert 1 galaxies, with the high-energy (inverse-Compton) emission dominating by more than an order of magnitude over the low-energy (synchrotron) emission during γ-ray flares. The difference between the two SED humps is smaller during the low-activity state. Fermi observations reveal a strong correlation between the γ-ray flux and spectral index, with the hardest spectrum observed during the brightest γ-ray state. The γ-ray flares occurred before and during a slow rising trend in the radio, but no direct association between γ-ray and radio flares could be established.

Conclusions. If the γ-ray flux is a mixture of synchrotron self-Compton and external Compton emission, the observed GeV spectral variability may result from varying relative contributions of these two emission components. This explanation fits the observed changes in the overall IR to γ-ray SED.