Non-thermal Emissions from Cool Cores Heated by Cosmic-Rays in Galaxy Clusters

TL;DR

This paper models non-thermal emissions in galaxy cluster cool cores heated by cosmic rays, finding steep CR spectra and predicting radio mini-halos, while noting challenges in detecting X-ray and gamma-ray emissions.

Contribution

It introduces a model where cosmic rays heat cluster cores via streaming, linking CR spectra to shock properties and predicting observable radio mini-halos.

Findings

CR spectra are steep, dominated by low-energy CRs.

Synchrotron radio mini-halos are expected in cluster cores.

Detection of non-thermal X-ray and gamma-ray emissions is challenging.

Abstract

We study non-thermal emissions from cool cores in galaxy clusters. We adopted a recent model, in which cosmic-rays (CRs) prevail in the cores and stably heat them through CR streaming. The non-thermal emissions come from the interaction between CR protons and intracluster medium (ICM). Comparison between the theoretical predictions and radio observations shows that the overall CR spectra must be steep, and most of the CRs in the cores are low-energy CRs. Assuming that the CRs are injected through AGN activities, we study the nature of the shocks that are responsible for the CR acceleration. The steep CR spectra are likely to reflect the fact that the shocks travel in hot ICM with fairly small Much numbers. We also study the dependence on the CR streaming velocity. The results indicate that synchrotron emissions from secondary electrons should be observed as radio mini-halos in the cores. In particular, low-frequency observations (e.g. LOFAR) are promising. On the other hand, the steepness of the spectra makes it difficult to detect non-thermal X-ray and gamma-ray emissions from the cores. The low-energy CRs may be heating optical filaments observed in the cores.