Numerical modelling of gamma radiation from galaxy clusters

Abstract

We investigate the spatial and spectral properties of non-thermal emission from clusters of galaxies at γ-ray energies. We estimate the radiation flux between 10 keV and 10 TeV due to inverse-Compton (IC) emission, π 0 -decay and non-thermal bremsstrahlung (NTB) from cosmic ray (CR) ions and electrons accelerated at cosmic shocks as well as secondary e ± generated in inelastic p-p collisions. We identify two main region of production of non-thermal radiation, namely the core (also bright in the thermal X-ray range) and the outskirts region where accretion shocks occur. We find that IC emission from shock-accelerated CR electrons dominate the emission at the outer regions of galaxy clusters, provided that at least a fraction of a per cent of the shock ram pressure is converted into CR electrons. A clear detection of this component and of its spatial distribution will allow us to probe the cosmic accretion shocks directly. In the cluster core, γ-ray emission above 100 MeV is dominated by the π0-decay mechanism. At lower energies, IC emission from secondary e± takes over. However, IC emission from shock-accelerated electrons projected on to the cluster core will not be negligible. We emphasize the importance of separating the aforementioned emission components for a correct interpretation of the experimental data and outline a strategy for that purpose. Failure to addresses this issue will produce unsound estimates of the intracluster magnetic field strength and CR ion content. According to our estimate future spaceborne and ground-based γ-ray facilities should be able to measure the whole non-thermal spectrum both in the cluster core and at its outskirts. The importance of such measurements in advancing our understanding of non-thermal processes in the intracluster medium is discussed.