The challenge of turbulent acceleration of relativistic particles in the intra-cluster medium

TL;DR

This paper investigates the physics of turbulent acceleration of cosmic-ray electrons in galaxy clusters' intra-cluster medium, highlighting uncertainties in plasma microphysics and their impact on non-thermal phenomena like radio halos.

Contribution

It analyzes how electromagnetic fluctuation spectra, damping, and mean-free-path uncertainties affect turbulent acceleration efficiency in the ICM.

Findings

Acceleration efficiency is highly sensitive to plasma microphysics.

Uncertainties in fluctuation spectra and damping mechanisms impact cosmic-ray acceleration.

Non-thermal cluster properties probe the microphysics of the ICM.

Abstract

Acceleration of cosmic-ray electrons (CRe) in the intra-cluster-medium (ICM) is probed by radio observations that detect diffuse, Mpc-scale, synchrotron sources in a fraction of galaxy clusters. Giant radio halos are the most spectacular manifestations of non-thermal activity in the ICM and are currently explained assuming that turbulence driven during massive cluster-cluster mergers reaccelerates CRe at several GeV. This scenario implies a hierarchy of complex mechanisms in the ICM that drain energy from large-scales into electromagnetic fluctuations in the plasma and collisionless mechanisms of particle acceleration at much smaller scales. In this paper we focus on the physics of acceleration by compressible turbulence. The spectrum and damping mechanisms of the electromagnetic fluctuations, and the mean-free-path (mfp) of CRe are the most relevant ingredients that determine the efficiency of acceleration. These ingredients in the ICM are however poorly known and we show that calculations of turbulent acceleration are also sensitive to these uncertainties. On the other hand this fact implies that the non-thermal properties of galaxy clusters probe the complex microphysics and the weakly collisional nature of the ICM.