Statistics of Giant Radio Halos from Electron Reacceleration Models

Abstract

The most important evidence of non-thermal phenomena in galaxy clusters comes from Giant Radio Halos (GRHs), synchrotron radio sources extended over Mpc scales, detected in a growing number of massive galaxy clusters. A promising possibility to explain these sources is given by "in situ" stochastic reacceleration of relativistic electrons by turbulence generated in the cluster volume during merger events. Cassano & Brunetti (2005) have recently shown that the expected fraction of clusters with GRHs and the increase of such a fraction with cluster mass can be reconciled with present observations provided that a fraction of 20-30 % of the turbulence in clusters is in the form of compressible modes. In this work we extend these calculations by including a scaling of the magnetic field strength with cluster mass. We show that the observed correlations between the synchrotron radio power of a sample of 17 GRHs and the X-ray properties of the hosting clusters are consistent with, and actually predicted by a magnetic field dependence on the virial mass of the form B \propto M^b, with b>0.5 and typical micro Gauss strengths of the average B intensity. The occurrence of GRHs as a function of both cluster mass and redshift is obtained. The most relevant findings are that the predicted luminosity functions of GRHs are peaked around a power P_{1.4 GHz} 10^{24} W/Hz, and severely cut-off at low radio powers due to the decrease of the electron reacceleration in smaller galaxy clusters. We expect a total number of GRHs to be discovered at ~mJy radio fluxes of ~100 at 1.4 GHz. Finally, the occurrence of GRHs and their number counts at 150 MHz are estimated in view of the fortcoming operation of low frequency observatories (LOFAR, LWA) and compared with those at higher radio frequencies.