Detection of very high energy gamma-ray emission from the Perseus cluster head-tail galaxy IC 310 by the MAGIC telescopes

TL;DR

This paper reports the first detection of very high energy gamma-ray emission from the galaxy IC 310 in the Perseus cluster by MAGIC telescopes, revealing variable emission likely originating from the inner jet.

Contribution

First detection of VHE gamma-ray emission from IC 310, with detailed spectral analysis and evidence of variability, suggesting complex emission models beyond simple SSC scenarios.

Findings

Detection at 7.6 sigma significance in 20.6 hours of data.

Flux above 300 GeV is about 2.5% of Crab Nebula.

Evidence of variability with possible flares in 2009.

Abstract

We report on the detection with the MAGIC telescopes of very high energy gamma-rays from IC 310, a head-tail radio galaxy in the Perseus galaxy cluster, observed during the interval November 2008 to February 2010. The Fermi satellite has also detected this galaxy. The source is detected by MAGIC at a high statistical significance of 7.6sigma in 20.6 hr of stereo data. The observed spectral energy distribution is flat with a differential spectral index of -2.00 \pm 0.14. The mean flux above 300 GeV, between October 2009 and February 2010, (3.1 \pm 0.5)x10^{-12} cm^{-2} s^{-1}, corresponds to (2.5 \pm 0.4)% of Crab Nebula units. Only an upper limit, of 1.9% of Crab Nebula units above 300 GeV, was obtained with the 2008 data. This, together with strong hints (>3sigma) of flares in the middle of October and November 2009, implies that the emission is variable. The MAGIC results favour a scenario with the very high energy emission originating from the inner jet close to the central engine. More complicated models than a simple one-zone SSC scenario, e.g. multi-zone SSC, external Compton or hadronic, may be required to explain the very flat spectrum and its extension over more than three orders of magnitude in energy.