Rapid and multiband variability of the TeV bright active nucleus of the galaxy IC 310

¹ IFAE, Edifici Cn., Campus UAB, 08193 Bellaterra, Spain
² INAF National Institute for Astrophysics, 00136 Rome, Italy
³ Università di Siena, and INFN Pisa, 53100 Siena, Italy
⁴ Croatian MAGIC Consortium, Rudjer Boskovic Institute, University of Rijeka and University of Split, 10000 Zagreb, Croatia
⁵ Max-Planck-Institut für Physik, 80805 München, Germany
⁶ Universidad Complutense, 28040 Madrid, Spain
⁷ Inst. de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁸ University of Łódź, 90236 Lodz, Poland
⁹ Depto. de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Spain
¹⁰ Deutsches Elektronen-Synchrotron (DESY), 15738 Zeuthen, Germany
¹¹ ETH Zürich, 8093 Zürich, Switzerland
¹² Universität Würzburg, 97074 Würzburg, Germany
¹³ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, 28040 Madrid, Spain
¹⁴ Università di Udine, and INFN Trieste, 33100 Udine, Italy
¹⁵ Technische Universität Dortmund, 44221 Dortmund, Germany
¹⁶ Inst. de Astrofísica de Andalucía (CSIC), 18080 Granada, Spain
¹⁷ Università di Padova and INFN, 35131 Padova, Italy
¹⁸ Unitat de Física de les Radiacions, Departament de Física, and CERES-IEEC, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
¹⁹ Università dell’Insubria, 22100 Como, Italy
²⁰ Institut de Ciències de l’Espai (IEEC-CSIC), 08193 Bellaterra, Spain
²¹ Tuorla Observatory, University of Turku, 21500 Piikkiö, Finland
²² Japanese MAGIC Consortium, Division of Physics and Astronomy, Kyoto University, Kahiwa, 277-8582 Chiba, Japan
²³ Inst. for Nucl. Research and Nucl. Energy, 1784 Sofia, Bulgaria
²⁴ Universitat de Barcelona (ICC/IEEC), 08028 Barcelona, Spain
²⁵ INAF/Osservatorio Astronomico and INFN, 34143 Trieste, Italy
²⁶ Università di Pisa, and INFN Pisa, 56126 Pisa, Italy
²⁷ ICREA, 08010 Barcelona, Spain
²⁸ Now at Ecole polytechnique fédérale de Lausanne (EPFL), 1015, Lausanne, Switzerland
²⁹ Supported by INFN Padova  
³⁰ Now at Department of Physics & Astronomy, UC Riverside, CA 92521, USA
³¹ Now at Finnish Centre for Astronomy with ESO (FINCA), University of Turku, 2014 Turku, Finland
³² Also at Instituto de Física Teorica, UAM/CSIC, 28049 Madrid, Spain
³³ Now at GRAPPA Institute, University of Amsterdam, 1098XH Amsterdam, The Netherlands
³⁴ Dr-Karl-Remeis-Observatory and Erlangen Centre for Astroparticle Physics, 96049 Bamberg, Germany
³⁵ Fred Lawrence Whipple Observatory, Harvard-Smithsonian Center for Astrophysics, Amado AZ 85645, USA

Received: 22 May 2013
Accepted: 23 December 2013

Abstract

Context. The radio galaxy IC 310 has recently been identified as a γ-ray emitter based on observations at GeV energies with Fermi-LAT and at very high energies (VHE, E > 100 GeV) with the MAGIC telescopes. Originally classified as a head-tail radio galaxy, the nature of this object is subject of controversy since its nucleus shows blazar-like behavior.

Aims. To understand the nature of IC 310 and the origin of the VHE emission, we studied the spectral and flux variability of IC 310 from the X-ray band to the VHE γ-ray regime.

Methods. The light curve of IC 310 above 300 GeV has been measured with the MAGIC telescopes from 2009 October to 2010 February. Contemporaneous Fermi-LAT data (2008−2011) in the 10−500 GeV energy range were also analyzed. In the X-ray regime, archival observations from 2003 to 2007 with XMM-Newton, Chandra, and Swift-XRT in the 0.5−10 keV band were studied.

Results. The VHE light curve reveals several high-amplitude and short-duration flares. Day-to-day flux variability is clearly present (>5σ). The photon index between 120 GeV and 8 TeV remains at the value Γ ~ 2.0 during both low and high flux states. The VHE spectral shape does not show significant variability, whereas the flux at 1 TeV changes by a factor of ~7. Fermi-LAT detected only eight γ-ray events in the energy range 10 GeV–500 GeV in three years of observation. The measured photon index of Γ = 1.3 ± 0.5 in the Fermi-LAT range is very hard. The X-ray measurements show strong variability in both flux and photon index. The latter varied from 1.76 ± 0.07 to 2.55 ± 0.07.

Conclusions. The rapid variability measured in γ-rays and X-rays confirms the blazar-like behavior of IC 310. The multi-TeV γ-ray emission seems to originate from scales of less than 80 Schwarzschild radii (for a black hole mass of 2 × 10⁸ M_⊙) within the compact core of its FR I radio jet with orientation angle 10°−38°. The spectral energy distribution resembles that of an extreme blazar, albeit the luminosity is more than two orders of magnitude lower.