Rapid and multiband variability of the TeV bright active nucleus of the galaxy IC 310
Maraschi, L.; Covino, S.; Antonelli, L. A.; Prada Moroni, P. G.; Giavitto, G.; Hadasch, D.; Paneque, D.; Tibolla, O.; Torres, D. F.; Nowak, N.; Tavecchio, F.; Herrero, A.; Treves, A.; Wilms, J.; Shore, S. N.; Stamerra, A.; Kadler, M.; Marcote, B.; Biland, A.; Garczarczyk, M.; Gaug, M.; Teshima, M.; Paredes, J. M.; Wagner, R. M.; Antoranz, P.; Babic, A.; Barres de Almeida, U.; Barrio, J. A.; Becerra González, J.; Bednarek, W.; Bernardini, E.; Blanch, O.; Bonnefoy, S.; Bonnoli, G.; Borracci, F.; Bretz, T.; Carosi, A.; Colin, P.; Colombo, E.; Contreras, J. L.; Cortina, J.; Da Vela, P.; Dazzi, F.; De Angelis, A.; De Lotto, B.; Doert, M.; Domínguez, A.; Dominis Prester, D.; Dorner, D.; Doro, M.; Elsaesser, D.; Fonseca, M. V.; Font, L.; Fruck, C.; García López, R. J.; Godinović, N.; Hose, J.; Hrupec, D.; Idec, W.; Kushida, J.; Lelas, D.; Lindfors, E.; Lombardi, S.; López, M.; López-Coto, R.; Makariev, M.; Mallot, K.; Maneva, G.; Mannheim, K.; Mariotti, M.; Martínez, M.; Mazin, D.; Mirzoyan, R.; Moralejo, A.; Nakajima, D.; Niedzwiecki, A.; Nilsson, K.; Paiano, S.; Palatiello, M.; Paoletti, R.; Persic, M.; Prandini, E.; Puljak, I.; Reichardt, I.; Rhode, W.; Ribó, M.; Rico, J.; Satalecka, K.; Schweizer, T.; Sillanpää, A.; Sitarek, J.; Snidaric, I.; Sobczynska, D.; Surić, T.; Takalo, L.; Temnikov, P.; Terzić, T.; Tescaro, D.; Toyama, T.; Zanin, R.; MAGIC Collaboration; Rügamer, S.; Masbou, J.; Spanier, F.; Weitzel, Q.; Prada, F.; Reinthal, R.; López-Oramas, A.; Miranda, J. M.; Munar-Adrover, P.; Aleksić, J.; Berger, K.; Bock, R. K.; Carmona, E.; Carreto Fidalgo, D.; De Caneva, G.; Delgado Mendez, C.; Eisenacher, D.; Farina, E.; Ferenc, D.; Garrido Terrats, D.; González Muñoz, A.; Gozzini, S. R.; Hadamek, A.; Kadenius, V.; Knoetig, M. L.; Krause, J.; La Barbera, A.; Lewandowska, N.; Lorenz, E.; Lozano, I.; Mankuzhiyil, N.; Meucci, M.; Orito, R.; Overkemping, A.; Partini, S.; Preziuso, S.; Saggion, A.; Saito, K.; Salvati, M.; Scalzotto, V.; Scapin, V.; Schultz, C.; Stamatescu, V.; Storz, J.; Sun, S.; Thaele, J.; Uellenbeck, M.; Vogler, P.; Zandanel, F.; Takami, H.; Boller, A.; Borla Tridon, D.; Cossio, L.; Häfner, D.; Krähenbühl, T.; Moldón, J.; Saito, T. Y.; Spiro, S.; Steinke, B.; Krauss, F.; Wilbert, S.
Spain, Italy, Croatia, Germany, Poland, Switzerland, United States, Finland, Japan, Bulgaria, Netherlands
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.