Cosmic Ray Streaming in Clusters of Galaxies

TL;DR

This paper explores how cosmic-ray streaming in galaxy clusters can suppress radio and gamma-ray emissions, explaining observed bimodality and predicting the evolution of radio halos and gamma-ray signals over time.

Contribution

It introduces a model where CR streaming depends on plasma parameters and wave damping mechanisms, providing new insights into the suppression of radio and gamma-ray emissions in galaxy clusters.

Findings

Radio halos diminish by an order of magnitude in hundreds of Myr.

Spectral curvature develops due to energy-dependent CR streaming.

Gamma-ray emission at high energies is rapidly suppressed.

Abstract

The observed bimodality in radio luminosity in galaxy clusters is puzzling. We investigate the possibility that cosmic-ray (CR) streaming in the intra-cluster medium can 'switch off' hadronically induced radio and gamma-ray emission. For self-confined CRs, this depends on the source of MHD wave damping: if only non-linear Landau damping operates, then CRs stream on the slow Alfvenic timescale, but if turbulent wave damping operates, super-Alfvenic streaming is possible. As turbulence increases, it promotes outward streaming more than it enables inward turbulent advection. Curiously, the CR flux is independent of $\nabla f$ (as long as it is non-zero) and depends only on plasma parameters; this enables radio halos with flat inferred CR profiles to turn off. We perform 1D time-dependent calculations of a radio mini-halo (Perseus) and giant radio halo (Coma) and find that both diminish in radio luminosity by an order of magnitude in several hundred Myr, given plausible estimates for the magnetic field in the outskirts of the cluster. Due to the energy dependence of CR streaming, spectral curvature develops, and radio halos turn off more slowly at low frequencies -- properties consistent with observations. Similarly, CR streaming rapidly turns off gamma-ray emission at the high-energies probed by Cherenkov telescopes, but not at the low energies probed by Fermi. CR mediated wave-heating of the ICM is unaffected, as it is dominated by ~GeV CRs which stream Alfvenically.