Acceleration of primary and secondary particles in galaxy clusters by compressible MHD turbulence: from radio halos to gamma rays

TL;DR

This paper develops a comprehensive model of turbulence-driven particle acceleration in galaxy clusters, explaining radio halos and gamma-ray emissions, and aligning well with current observations and constraints.

Contribution

It extends previous models by self-consistently including reacceleration of primary and secondary particles in the IGM using advanced MHD turbulence theory.

Findings

Radio to gamma-ray emission spectra depend on cluster dynamics.

Giant radio halos form only in merging, turbulent clusters.

Relaxed clusters may have detectable synchrotron emission near current upper limits.

Abstract

Radio observations discovered large scale non thermal sources in the central Mpc regions of dynamically disturbed galaxy clusters (radio halos). The morphological and spectral properties of these sources suggest that the emitting electrons are accelerated by spatially distributed and gentle mechanisms, providing some indirect evidence for turbulent acceleration in the inter-galactic-medium (IGM). Only deep upper limits to the energy associated with relativistic protons in the IGM have been recently obtained through gamma and radio observations. Yet these protons should be (theoretically) the main non-thermal particle component in the IGM implying the unavoidable production, at some level, of secondary particles that may have a deep impact on the gamma ray and radio properties of galaxy clysters. Following Brunetti and Lazarian (2007), in this paper we consider the advances in the theory of MHD turbulence to develop a comprehensive picture of turbulence in the IGM and extend our previous calculations of particle acceleration by compressible MHD turbulence by considering self-consistently the reacceleration of both primary and secondary particles. Under these conditions we expect that radio to gamma ray emission is generated from galaxy clusters with a complex spectrum that depends on the dynamics of the thermal gas and Dark Matter. The non-thermal emission results in very good agreement with radio observations and with present constraints from hard X-ray and gamma ray observations. In our model giant radio halos are generated in merging (turbulent) clusters only. However, in case secondaries dominate the electron component in the IGM, we expect that the level of the Mpc-scale synchrotron emission in more relaxed clusters is already close to that of the radio upper limits derived by present observations of clusters without radio halos.