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Cosmic gamma rays from galaxy clusters

Recent analysis of the data base of cosmic gamma rays above 70 MeV from the COS B satellite by Strong and Bignami (1983) has led to the detection of a flux of approximately 8*10-7 cm-2 s-1 at Earth from the direction of NGC 1275 in the Perseus cluster of galaxies. The authors have gone back to the database of the SAS II experiment given by Fichtel et al. (1978) and looked for the weak signal from galaxy clusters out to a distance of about 590 Mpc, employing a statistical technique. A significant flux is detected, in both the regions 35-100 MeV and above 100 MeV, and the average flux per cluster (the clusters being of 'richness category' 1 and above) corresponds to about 3*10-7 cm-2 s-1 above 10 MeV, per cluster at the Perseus distance, i.e. about 35% of the value given by Strong and Bignami. Adopting astronomical data for the space density of clusters and neglecting cosmological effects, the contribution of clusters, out to the Hubble radius, the intensity of extragalactic gamma rays is approximately 5*10-5 cm-2 s-1 sr-1 above 35 MeV, which is rather close to the total extragalactic gamma-ray intensity detected by Fichtel and Thompson (1982). The contribution from galaxy clusters to the extragalactic gamma-ray flux is thus important.