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4. CLUSTERS OF GALAXIES

Nonthermal radiation from clusters of galaxies is expected for several reasons: cosmic rays will be accelerated through merger shocks from merging subclusters, from accretion shocks as primordial matter continues to accrete on a forming cluster, and from turbulent reacceleration of nonthermal particles by plasma waves in the intracluster medium. In addition, a galaxy cluster often has an energetic AGN in its central cD galaxy that could inject cosmic rays into the cluster medium. Hadronic cosmic rays with energies ltapprox 1019 eV will be trapped on timescales longer than the Hubble time, so galaxy clusters become storage volumes for cosmic rays [4]. In spite of these expectations, EGRET did not make a high-significance detection of any galaxy cluster [36].

Hard X-ray tails have also not been detected with high significance from the Coma cluster or any other galaxy cluster. The study of nonthermal emission from clusters of galaxies has consequently stalled, as nonthermal X-ray measurements provide the crucial information to normalize the magnetic field and nonthermal electron spectrum. Predictions based on the marginal detection of the hard X-ray tail from the Coma cluster indicate that Coma will be easily detectable with GLAST in one year of observation and marginally detectable with ground-based gamma-ray telescopes in a nominal 50 hour observation [5], though the angular extent of Coma makes such detections more difficult [17].

In view of these uncertainties, any calculation of the integrated contribution from clusters of galaxies to the gamma-ray background is likewise highly uncertain. Fig. 1a shows predictions [21, 6] for galaxy cluster emission. GLAST detections of clusters of galaxies will be crucial to provide a better basis for determining this contribution.

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