Gamma rays from galaxy clusters

Abstract

Galaxy clusters are expected to be sites of particle acceleration and sources of non-thermal radiation from radio to gamma-rays. Non-thermal components in clusters are unique probes of complex mechanisms operating in cluster volumes that drain gravitational and electromagnetic energy into cosmic rays (CR) and magnetic fields. These processes are currently best probed by radio observations that detect cluster-scale diffuse synchrotron radiation from clusters. Interestingly galaxy clusters are unique astrophysical laboratories since their large sizes allow to confine high energy particles for very long times inducing a continuous production of secondary particles, including neutral pions that decay into gamma-rays.In this review we will summarize the basic motivations that lead to predict gamma-ray emission from galaxy clusters, the results from current observations and their implications for the acceleration and dynamics of cosmic rays in galaxy clusters. Meaningful expectations for gamma-ray emission from galaxy clusters will be briefly discussed. In particular we will show that, under the favourable hypothesis that giant radio halos in galaxy clusters originate from turbulent reacceleration of secondary particles, future gamma-ray observations have the potential to obtain the first detections of nearby massive clusters.