Hard X-ray emission from accretion shocks around galaxy clusters

TL;DR

This paper explains the observed hard X-ray emission in galaxy clusters as inverse Compton scattering by electrons accelerated in accretion shocks, aligning well with observed data and predicting future observational signatures.

Contribution

It introduces a simple model linking HXR emission to accretion shocks and relativistic electrons, providing a better fit to observations than alternative models.

Findings

HXR surface brightness depends on cluster temperature as predicted

Observed HXR luminosity consistent with less than 0.1% shock energy in electrons

Disfavors models correlating HXR with thermal emission

Abstract

We show that the hard X-ray (HXR) emission observed from several galaxy clusters is naturally explained by a simple model, in which the nonthermal emission is produced by inverse Compton scattering of cosmic microwave background photons by electrons accelerated in cluster accretion shocks: The dependence of HXR surface brightness on cluster temperature is consistent with that predicted by the model, and the observed HXR luminosity is consistent with the fraction of shock thermal energy deposited in relativistic electrons being \lesssim 0.1. Alternative models, where the HXR emission is predicted to be correlated with the cluster thermal emission, are disfavored by the data. 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.