Statistics of Giant Radio Halos from Electron Reacceleration Models

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.Comment: 21 pages, 17 figures, accepted for publication in MNRA

Particle reacceleration in Coma cluster: radio properties and hard X-ray emission

The radio spectral index map of the Coma halo shows a progressive steepening of the spectral index with increasing radius. Such a steepening cannot be simply justified by models involving continuous injection of fresh particles in the Coma halo or by models involving diffusion of fresh electrons from the central regions. We propose a {\it two phase} model in which the relativistic electrons injected in the Coma cluster by some processes (starbursts, AGNs, shocks, turbulence) during a {\it first phase} in the past are systematically reaccelerated during a {\it second phase} for a relatively long time ($\sim$ 1 Gyr) up to the present time. We show that for reacceleration time scales of $\sim 0.1$ Gyr this hypothesis can well account for the radio properties of Coma C. For the same range of parameters which explain Coma C we have calculated the expected fluxes from the inverse Compton scattering of the CMB photons finding that the hard X-ray tail discovered by BeppoSAX may be accounted for by the stronger reacceleration allowed by the model. The possibility of extending the main model assumptions and findings to the case of the other radio haloes is also discussed, the basic predictions being consistent with the observations.Comment: 15 pages, 13 figures, accepted for publication in MNRA

The contribution of AGN to the X-ray background: the effect of iron features

The contribution of the iron emission line, commonly detected in the X-ray spectra of Seyfert (Sey) galaxies, to the cosmic X-ray background (XRB) spectrum is evaluated in the framework of the XRB synthesis models based on AGN unification schemes. To derive the mean line properties, we have carried out a search in the literature covering a sample of about 70 AGN. When adopting line parameters in agreement with the observations, it turns out that the maximum contribution of the iron line to the XRB is less than 7% at a few keV. This is still below the present uncertainties in the XRB spectrum measurements.Comment: 21 LaTeX pages with 5 Postscript figures. Accepted for publication in New Astronom