Giant radio halos in galaxy clusters as probes of particle acceleration in turbulent regions

TL;DR

Giant radio halos in galaxy clusters serve as tools to study particle acceleration mechanisms driven by turbulence during cluster mergers, with models aligning well with current observations but still facing unresolved questions.

Contribution

This paper reviews the physics of turbulent (re)acceleration of particles in galaxy clusters and discusses how future observations can test these models.

Findings

Models of turbulent acceleration match current radio and gamma-ray observations.

Turbulent reacceleration can explain the properties of giant radio halos.

Open problems remain in fully understanding particle acceleration processes.

Abstract

Giant radio halos in galaxy clusters probe mechanisms of particle acceleration connected with cluster merger events. Shocks and turbulence are driven in the inter-galactic-medium (IGM) during clusters mergers and may have a deep impact on the non-thermal properties of galaxy clusters. Models of turbulent (re)acceleration of relativistic particles allow good correspondence with present observations, from radio halos to gamma-ray upper limits, although several aspects of this complex scenario remain still poorly understood. After providing basic motivations for turbulent acceleration in galaxy clusters, we discuss relevant aspects of the physics of particle acceleration by MHD turbulence and the expected broad--band non-thermal emission from galaxy clusters. We discuss (in brief) the most important results of turbulent (re)acceleration models, the open problems, and the possibilities to test models with future observations. In this respect, further constraints on the origin of giant nearby radio halos can also be obtained by combining their (spectral and morphological) properties with the constraints from gamma-ray observations of their parent clusters.