Detection of very-high energy gamma-ray emission from NGC 1275 by the MAGIC telescopes

We report on the detection of very-high energy (VHE, E > 100 GeV) gamma-ray emission from <ASTROBJ>NGC 1275</ASTROBJ>, the central radio galaxy of the Perseus cluster of galaxies. The source has been detected by the MAGIC telescopes with a statistical significance of 6.6sigma above 100 GeV in 46 h of stereo observations carried out between August 2010 and February 2011. The measured differential energy spectrum between 70 GeV and 500 GeV can be described by a power law with a steep spectral index of Gamma = -4.1 0.7<SUB>stat</SUB> 0.3<SUB>syst</SUB>, and the average flux above 100 GeV is F<SUB>gamma</SUB> = (1.3 0.2<SUB>stat</SUB> 0.3<SUB>syst</SUB>) 10<SUP>-11</SUP> cm<SUP>-2</SUP> s<SUP>-1</SUP>. These results, combined with the power-law spectrum measured in the first two years of observations by the Fermi-LAT above 100 MeV, with a spectral index of Gamma ~= - 2.1, strongly suggest the presence of a break or cut-off around tens of GeV in the <ASTROBJ>NGC 1275</ASTROBJ> spectrum. The light curve of the source above 100 GeV does not show hints of variability on a month time scale. Finally, we report on the nondetection in the present data of the radio galaxy <ASTROBJ>IC 310</ASTROBJ>, previously discovered by the Fermi-LAT and MAGIC. The derived flux upper limit F<SUP>U.L.</SUP><SUB>gamma</SUB> (>300 GeV) = 1.2 10<SUP>-12</SUP> cm<SUP>-2</SUP> s<SUP>-1</SUP> is a factor ~ 3 lower than the mean flux measured by MAGIC between October 2009 and February 2010, thus confirming the year time-scale variability of the source at VHE.