MAGIC Gamma-Ray Observation of the Perseus Galaxy Cluster
Abstract
The Perseus galaxy cluster was observed by the MAGIC Cherenkov telescope for a total effective time of 24.4 hours during November and December 2008. The resulting upper limits on the gamma-ray emission above 100 GeV are in the range of 4.6 to 7.5 x 10^{-12} cm^{-2} s^{-1} for spectral indices from -1.5 to -2.5; thereby constraining the emission produced by cosmic rays, dark matter annihilations, and the central radio galaxy NGC 1275. Results are compatible with cosmological cluster simulations for the cosmic ray induced gamma-ray emission, constraining the relative cosmic ray pressure to <3.5% for the cluster core region (<7% when considering biasing effects of galaxy-sized substructures). Using simplified assumptions adopted in earlier work (constant cosmic ray-to-thermal pressure throughout the entire cluster and a power-law spectrum with an index of -2.1), we constrain the ratio of cosmic ray-to-thermal energy to E_{CR}/E_{th}<1.7%. The upper limit also translates to a level of gamma-ray emission from possible annihilations of the cluster dark matter (the dominant mass component) that is consistent with boost factors of ~10^{4} for the typically expected dark matter annihilation induced emission. Finally, the upper limits obtained for the gamma-ray emission of the central radio galaxy NGC 1275 are consistent with the recent detection by the Fermi-LAT satellite. Due to the extremely large Doppler factors required for the jet, a one-zone synchrotron self-Compton model is implausible in this case. We reproduce the observed spectral energy density by using the structured jet (spine-layer) model which has previously been adopted to explain the high-energy emission of radio galaxies.Keywords
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