MAGIC GAMMA-RAY TELESCOPE OBSERVATION OF THE PERSEUS CLUSTER OF GALAXIES: IMPLICATIONS FOR COSMIC RAYS, DARK MATTER, AND NGC 1275

Abstract

The Perseus galaxy cluster was observed by the MAGIC Cherenkov telescope for\na total effective time of 24.4 hr during 2008 November and December. The\nresulting upper limits on the gamma-ray emission above 100 GeV are in the range\nof 4.6 to 7.5 x 10^{-12} cm^{-2} s^{-1} for spectral indices from -1.5 to -2.5,\nthereby constraining the emission produced by cosmic rays, dark matter\nannihilations, and the central radio galaxy NGC1275. Results are compatible\nwith cosmological cluster simulations for the cosmic-ray-induced gamma-ray\nemission, constraining the average cosmic ray-to-thermal pressure to <4% for\nthe cluster core region (<8% for the entire cluster). Using simplified\nassumptions adopted in earlier work (a power-law spectrum with an index of\n-2.1, constant cosmic ray-to-thermal pressure for the peripheral cluster\nregions while accounting for the adiabatic contraction during the cooling flow\nformation), we would limit the ratio of cosmic ray-to-thermal energy to\nE_CR/E_th<3%. The upper limit also translates into a level of gamma-ray\nemission from possible annihilations of the cluster dark matter (the dominant\nmass component) that is consistent with boost factors of ~10^4 for the\ntypically expected dark matter annihilation-induced emission. Finally, the\nupper limits obtained for the gamma-ray emission of the central radio galaxy\nNGC1275 are consistent with the recent detection by the Fermi-LAT satellite.\nDue to the extremely large Doppler factors required for the jet, a one-zone\nsynchrotron self-Compton model is implausible in this case. We reproduce the\nobserved spectral energy density by using the structured jet (spine-layer)\nmodel which has previously been adopted to explain the high-energy emission of\nradio galaxies.\n