Cosmic Rays and Radio Halos in Galaxy Clusters: New Constraints from Radio Observations

Clusters of galaxies are sites of acceleration of charged particles and sources of nonthermal radiation. We report on new constraints on the population of cosmic rays in the intracluster medium (ICM) obtained via radio observations of a fairly large sample of massive, X-ray-luminous galaxy clusters in the redshift interval 0.2-0.4. The bulk of the observed galaxy clusters does not show any hint of megaparsec-scale synchrotron radio emission at the cluster center (radio halo). We obtained solid upper limits to the diffuse radio emission and discuss their implications for the models for the origin of radio halos. Our measurements allow us to also derive a limit to the content of cosmic-ray protons in the ICM. Assuming spectral indices of these protons δ=2.1-2.4 and μG level magnetic fields, as from rotation measures, these limits are 1 order of magnitude deeper than present EGRET upper limits, while they are less stringent for steeper spectra and lower magnetic fields.