Two active states of the narrow-line gamma-ray-loud AGN GB 1310+487
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.
Additional Information© 2014 ESO. Article published by EDP Sciences. Received 7 November 2012; Accepted 8 January 2014. Published online 28 April 2014. We thank Sara Cutini, Marco Ajello, Denis Bastieri, Boris Komberg, Seth Digel, Luca Latronico and the anonymous referee for discussions and comments that helped improve this paper. The Fermi/LAT Collaboration acknowledges generous ongoing support from a number of agencies and institutes that have supported both the development and the operation of the LAT as well as scientific data analysis. These include the National Aeronautics and Space Administration (NASA) and the Department of Energy in the United States, the Commissariat à l'Energie Atomique and the Centre National de la Recherche Scientifique/Institut National de Physique Nucléaire et de Physique des Particules in France, the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), High Energy Accelerator Research Organization (KEK) and Japan Aerospace Exploration Agency (JAXA) in Japan, and the K. A. Wallenberg Foundation, and the Swedish Research Council as well as the Swedish National Space Board in Sweden. Additional support for science analysis during the operations phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica in Italy and the Centre National d'Études Spatiales in France. We acknowledge the use of public data from the Swift data archive at the High Energy Astrophysics Science Archive Research Center (HEASARC), provided by NASA's Goddard Space Flight Center. Based in part on observations with the 100m telescope of the MPIfR (Max-Planck-Institut für Radioastronomie) and the IRAM 30m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). The OVRO 40m monitoring program is supported in part by NASA grants NNX08AW31G and NNG06GG1G, and by NSF grant AST-0808050. This research has made use of data from the MOJAVE database that is maintained by the MOJAVE team (Lister et al. 2009a). The data presented herein were obtained in part with ALFOSC, which is provided by the Instituto de Astrofisica de Andalucia (IAA) under a joint agreement with the University of Copenhagen and NOTSA. The MOJAVE project is supported under NASA-Fermi grant NNX08AV67G. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We thank O. Fox, P. Kelly, I. Shivvers, and W. Zheng for assistance with some of the Keck observations. The near-IR observations were carried out with the 2.1m telescope of the Guillermo Haro Observatory, INAOE, Mexico. F.K.S. and K.V.S. were partly supported for this research. F.K.S. acknowledges support by the NASA Fermi Guest Investigator program, grant NXX12A075G. I.N. and R.S. are members of the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Universities of Bonn and Cologne. K.V.S., Y.A.K., and Y.Y.K. were supported in part by the Russian Foundation for Basic Research (Projects 11-02-00368 and 13-02-12103), the basic research program "Active processes in galactic and extragalactic objects" of the Physical Sciences Division of the Russian Academy of Sciences, and the Ministry of Education and Science of the Russian Federation (agreement No. 8405). Y.Y.K. was also supported by the Dynasty Foundation. RATAN-600 operations were carried out with the financial support of the Ministry of Education and Science of the Russian Federation (contract 14.518.11.7054). K.V.S. was supported by the Science Education Complex of the Lebedev Physical Inst. (UNK-FIAN). A.B.P. was supported by the "Non-stationary processes in the Universe" Program of the Presidium of the Russian Academy of Sciences. A.V.F. and S.B.C. are grateful for the support of NASA/Fermi grant NNX12AF12GA, NSF grant AST-1211916, the Christopher R. Redlich Fund, and Gary and Cynthia Bengier. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. We also used NASA's Astrophysics Data System. K.V.S. thanks Maria Mogilen for her help in preparing this manuscript.
Published - aa20703-12.pdf
Submitted - 1401.2151v5.pdf