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

We report on the detection of very-high energy (VHE, E > 100 GeV) γ-ray emission from NGC 1275, the central radio galaxy of the Perseus cluster of galaxies. The source has been detected by the MAGIC telescopes with a statistical significance of 6.6σ 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 Γ =-4.1 ± 0.7 stat ± 0.3syst, and the average flux above 100 GeV is Fγ = (1.3 ± 0.2stat ± 0.3 syst) × 10-11 cm-2 s-1. 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 Γ ≈-2.1, strongly suggest the presence of a break or cut-off around tens of GeV in the NGC 1275 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 IC 310, previously discovered by the Fermi-LAT and MAGIC. The derived flux upper limit F U.L.γ (>300 GeV) = 1.2 × 10-12 cm-2 s-1 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. © 2012 ESO.