Nonthermal phenomena in clusters of galaxies

TL;DR

Recent X-ray and radio observations suggest nonthermal phenomena in galaxy clusters, involving relativistic electrons and magnetic fields, but definitive evidence remains challenging due to observational uncertainties.

Contribution

This review synthesizes observational evidence for nonthermal emissions in galaxy clusters and discusses the implications for intracluster magnetic fields and relativistic electron populations.

Findings

Evidence of nonthermal X-ray emission in some clusters

Relativistic electrons inferred from radio and Faraday rotation data

Uncertainties hinder definitive conclusions about nonthermal processes

Abstract

Recent observations of high energy (> 20 keV) X-ray emission in a few clusters of galaxies broaden our knowledge of physical phenomena in the intracluster space. This emission is likely to be nonthermal, probably resulting from Compton scattering of relativistic electrons by the cosmic microwave background (CMB) radiation. Direct evidence for the presence of relativistic electrons in some 50 clusters comes from measurements of extended radio emission in their central regions. We briefly review the main results from observations of extended regions of radio emission, and Faraday rotation measurements of background and cluster radio sources. The main focus of the review are searches for nonthermal X-ray emission conducted with past and currently operating satellites, which yielded appreciable evidence for nonthermal emission components in the spectra of a few clusters. This evidence is clearly not unequivocal, due to substantial observational and systematic uncertainties, in addition to virtually complete lack of spatial information. If indeed the emission has its origin in Compton scattering of relativistic electrons by the CMB, then the mean magnetic field strength and density of relativistic electrons in the cluster can be directly determined. Knowledge of these basic nonthermal quantities is valuable for the detailed description of processes in intracluster gas and for the origin of magnetic fields.