Volume 563, March 2014
|Number of page(s)||9|
|Published online||14 March 2014|
Rapid and multiband variability of the TeV bright active nucleus of the galaxy IC 310
IFAE, Edifici Cn., Campus UAB,
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
Accepted: 23 December 2013
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.
Key words: galaxies: active / galaxies: individual: IC 310 / gamma rays: galaxies / X-rays: galaxies
© ESO, 2014
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