Shock Acceleration and gamma radiation in Clusters of Galaxies

The nonthermal radiation observed from a handfull of clusters of Galaxies is the proof that particle acceleration occurs in the intracluster medium. It is often believed that shock surfaces associated with either mergers of clusters of galaxies, or with the cosmological inflow of matter onto clusters during structure formation may be the sites for acceleration. We discuss here the effectiveness of shock acceleration in the intracluster medium, stressing that merger related shocks are typically weak, at least for the so-called major mergers. We investigate the implications of shock strengths for gamma ray emission from single clusters of galaxies and for their detectability with future gamma ray experiments such as GLAST. We also discuss the contribution of clusters of galaxies to the extragalactic diffuse gamma ray background. Nonthermal radiation is observed from several clusters of galaxies, showing that accelerated particles are present in the intracluster medium. Although the processes responsible for the acceleration are poorly known, there are some hints that they may be related to the merging processes that build up the clusters from smaller substructures. During cluster mergers, shock waves are formed due to the supersonic relative motion, and particle acceleration may take place through the first order Fermi process [1,2]. In [3], shock acceleration during cluster mergers was investigated without ad hoc assumptions on the strength of the shocks. The compression factors of these shocks were instead calculated from the physical properties of clusters involved in the process of hierarchical structure formation. In Fig. 1 we plot the distribution of Mach numbers of shocks formed during mergers that end up in a cluster of mass 10 15 M ⊙ at the present cosmic time. This plot shows that the large majority of mergers have Mach numbers below 2, which correspond to spectra of accelerated particles which are much steeper that those that can explain nonthermal observations in the radio band. Only ∼ 6% of the mergers have spectra flatter than E −2.4 (corresponding to Mach numbers larger than 3), which suggests that Fermi acceleration at merger shocks may not pp. 1–4 c 2003 by Universal Academy Press, Inc.