Nonthermal radiation mechanisms

TL;DR

This paper reviews non-thermal radiation mechanisms in galaxy clusters, focusing on radio and X-ray emissions, and discusses models involving synchrotron, inverse Compton, and bremsstrahlung processes, along with observational constraints.

Contribution

It provides an updated analysis of non-thermal emission models, including a more accurate treatment of bremsstrahlung and insights into magnetic field constraints from observations.

Findings

Alleviates the energy requirement for hard X-ray emission models.

Suggests low magnetic field values are needed to match observations.

Discusses prospects for gamma-ray detection with GLAST.

Abstract

In this paper we review the possible radiation mechanisms for the observed non-thermal emission in clusters of galaxies, with a primary focus on the radio and hard X-ray emission. We show that the difficulty with the non-thermal, non-relativistic Bremsstrahlung model for the hard X-ray emission, first pointed out by Petrosian (2001) using a cold target approximation, is somewhat alleviated when one treats the problem more exactly by including the fact that the background plasma particle energies are on average a factor of 10 below the energy of the non-thermal particles. This increases the lifetime of the non-thermal particles, and as a result decreases the extreme energy requirement, but at most by a factor of three. We then review the synchrotron and so-called inverse Compton emission by relativistic electrons, which when compared with observations can constrain the value of the magnetic field and energy of relativistic electrons. This model requires a low value of the magnetic field which is far from the equipartition value. We briefly review the possibilities of gamma-ray emission and prospects for GLAST observations. We also present a toy model of the non-thermal electron spectra that are produced by the acceleration mechanisms discussed in an accompanying paper.