High-energy gamma-ray and neutrino backgrounds from clusters of galaxies
  and radio constraints
  Cosmic-ray protons accumulate for cosmological times in clusters of galaxies
as their typical radiative and diffusive escape times are longer than the
Hubble time. Their hadronic interactions with protons of the intra-cluster
medium generate secondary electrons, gamma-rays and neutrinos. We here estimate
the contribution from clusters to the diffuse gamma-ray and neutrino
backgrounds. We model the cluster population by means of their mass function,
using a phenomenological luminosity-mass relation applied to all clusters, as
well as a detailed semi-analytical model. Additionally, we consider
observationally-motivated values for the cluster magnetic field. This is a
crucial parameter since the observed radio counts due to synchrotron emission
by secondary electrons need to be respected. For a choice of parameters
respecting all current constraints, and assuming a spectral index of -2, we
find that hadronic interactions in clusters contribute by less than 10% to the
extragalactic gamma-ray background observed by Fermi and to the IceCube flux.
They account for less than 1% for spectral indices <=-2. The neutrino flux
observed by IceCube can be reproduced without violating constraints only if a
very hard (and speculative) spectral index >-2 is adopted. However, this
scenario is in tension with the IceCube data, which seem to suggest a spectral
energy distribution of the neutrino flux that decreases with energy. In the
case of proton-photon interactions, we find that very likely protons do not
reach sufficiently high energies to produce neutrinos in clusters. We argue
that our results are optimistic due to our assumptions, and that clusters of
galaxies cannot give any relevant contribution to the extragalactic gamma-ray
and neutrino backgrounds. Finally, we find that the cluster contribution to the
angular fluctuations in the gamma-ray background is subdominant, less than 10%.
[abridged]
