Nonthermal emission from clusters of galaxies
  We show that the spectral and radial distribution of the nonthermal emission
of massive, M>10^{14.5}M_sun, galaxy clusters (GCs) may be approximately
described by simple analytic expressions, which depend on the GC thermal X-ray
properties and on two model parameter, beta_{core} and eta_e. beta_{core} is
the ratio of CR energy density (within a logarithmic CR energy interval) and
the thermal energy density at the GC core, and eta_{e(p)} is the fraction of
the thermal energy generated in strong collisionless shocks, which is deposited
in CR electrons (protons). Using a simple analytic model for the evolution of
ICM CRs, which are produced by accretion shocks (primary CRs), we find that
beta_{core} ~ eta_{p}/200, nearly independent of GC mass and with a scatter
Delta ln(beta_{core}) ~ 1 between GCs of given mass. We show that the HXR and
gamma-ray luminosities produced by IC scattering of CMB photons by primary
electrons exceed the luminosities produced by secondary particles (generated in
hadronic interactions within the GC) by factors ~500(eta_e/eta_p)(T/10
keV)^{-1/2} and ~150(eta_e/eta_p)(T/10 keV)^{-1/2} respectively, where T is the
GC temperature. Secondary particle emission may dominate at the radio and VHE
(> 1 TeV) gamma-ray bands. Our model predicts, in contrast with some earlier
work, that the HXR and gamma-ray emission from GCs are extended, since the
emission is dominated at these energies by primary electrons. Our predictions
are consistent with the observed nonthermal emission of the Coma cluster for
eta_peta_e ~ 0.1. The implications of our predictions to future HXR
observations (e.g. by NuStar, Simbol-X) and to (space/ground based) gamma-ray
observations (e.g. by Fermi, HESS, MAGIC, VERITAS) are discussed. Finally, we
show that our model's results agree with results of detailed numerical
calculations.
