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Context. The radio galaxy IC 310 has recently been identified as a gamma-rayemitter based on observations at GeV energies with Fermi-LAT and at very highenergies (VHE, E>100GeV) with the MAGIC telescopes. Originally classified as ahead-tail radio galaxy, the nature of this object is subject of controversysince its nucleus shows blazar-like behavior. Aims. In order to understand thenature of IC 310 and the origin of the VHE emission we studied the spectral andflux variability of IC 310 from the X-ray band to the VHE gamma-ray regime.Methods. The light curve of IC 310 above 300GeV has been measured with theMAGIC telescopes from Oct. 2009 to Feb. 2010. Fermi-LAT data (2008-2011) in the10-500GeV energy range were also analyzed. In X-ray, archival observations from2003 to 2007 with XMM, Chandra, and Swift-XRT in the 0.5-10keV band werestudied. Results. The VHE light curve reveals several high-amplitude andshort-duration flares. Day-to-day flux variability is clearly present. Thephoton index between 120GeV and 8TeV remains at the value $\Gamma\sim2.0$during both low and high flux states. The VHE spectral shape does not showsignificant variability, whereas the flux at 1TeV changes by a factor of$\sim7$. Fermi-LAT detected only eight gamma-ray events in the energy range10GeV-500GeV in three years of observation. The measured photon index of$\Gamma=1.3\pm0.5$ in the Fermi-LAT range is very hard. The X-ray measurementsshow strong variability in flux and photon index. The latter varied from$1.76\pm0.07$ to $2.55\pm0.07$. Conclusion. The rapid variability measuredconfirms the blazar-like behavior of IC 310. The TeV emission seems tooriginate from scales of less than 80 Schwarzschild radii within the compactcore of its FRI radio jet with orientation angle 10deg-38deg. The SED resemblesthat of an extreme blazar, albeit the luminosity is more than two orders ofmagnitude lower.

Rapid and multi-band variability of the TeV-bright active nucleus of the galaxy IC 310