Cosmic rays and Radio Halos in galaxy clusters : new constraints from radio observations

TL;DR

This study uses radio observations of galaxy clusters to set new constraints on cosmic ray populations and the origins of radio halos, with implications for cosmic ray physics and cluster magnetic fields.

Contribution

It provides the first large-sample radio constraints on cosmic rays in galaxy clusters, improving previous limits and informing models of radio halo formation.

Findings

Most clusters show no diffuse radio emission, setting upper limits.

Derived cosmic ray proton content limits are an order of magnitude deeper than previous gamma-ray limits.

Results depend on assumed spectral indices and magnetic field strengths.

Abstract

Clusters of galaxies are sites of acceleration of charged particles and sources of non-thermal radiation. We report on new constraints on the population of cosmic rays in the Intra Cluster 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 Mpc 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 derive also a limit to the content of cosmic ray protons in the ICM. Assuming spectral indices of these protons delta =2.1-2.4 and microG level magnetic fields, as from Rotation Measures, these limits are one order of magnitude deeper than present EGRET upper limits, while they are less stringent for steeper spectra and lower magnetic fields.