The Broadband Spectrum of Galaxy Clusters

TL;DR

This paper models the nonthermal proton-electron interactions in galaxy clusters, predicting a unique electron population that impacts radio and X-ray emissions, with future observations poised to test these predictions.

Contribution

It presents the first self-consistent kinetic model of nonthermal proton and thermal electron interactions in galaxy clusters, moving beyond the traditional knock-on approximation.

Findings

Electron population is out of equilibrium and not a power law.

Predicted bremsstrahlung efficiency exceeds that of a pure power law.

Model can be tested with upcoming GLAST observations.

Abstract

We examine whether nonthermal protons energized during a cluster merger are simultaneously responsible for the Coma cluster's diffuse radio flux (via secondary decay) and the departure of its intra-cluster medium (ICM) from a thermal profile via Coulomb collisions between the quasithermal electrons and the hadrons. Rather than approximating the influence of nonthermal proton/thermal electron collisions as extremely rare events which cause an injection of nonthermal, power-law electrons (the `knock-on' approximation), we self-consistently solve (to our knowledge, for the first time) the covariant kinetic equations for the two populations. The electron population resulting from these collisions is out of equilibrium, yet not a power law, and importantly displays a higher bremsstrahlung radiative efficiency than a pure power law. Observations with GLAST will test this model directly.