Entropy at the Outskirts of Galaxy Clusters as Implications for Cosmological Cosmic-Ray Acceleration

TL;DR

This paper investigates how entropy profiles at galaxy cluster outskirts can reveal the efficiency of cosmic-ray acceleration, finding that a significant fraction of kinetic energy is diverted into cosmic rays, affecting the entropy distribution.

Contribution

It introduces a novel method to use entropy profiles to constrain cosmic-ray acceleration efficiency in galaxy clusters.

Findings

Cosmic rays carry less than 7% of the kinetic energy at cluster outskirts.

Cosmic-ray pressure can be up to 40% of total pressure at outskirts.

Flat entropy profiles suggest efficient cosmic-ray acceleration.

Abstract

Recently, gas entropy at the outskirts of galaxy clusters attracts much attention. We propose that the entropy profiles could be used to study cosmic-ray (CR) acceleration around the clusters. If the CRs are effectively accelerated at the formation of clusters, the kinetic energy of infalling gas is consumed by the acceleration and the gas entropy should decrease. As a result, the entropy profiles become flat at the outskirts. If the acceleration is not efficient, the entropy should continue to increase outwards. By comparing model predictions with X-ray observations with Suzaku, which show flat entropy profiles, we find that the CRs have carried ~<7% of the kinetic energy of the gas away from the clusters. Moreover, the CR pressure at the outskirts can be ~<40% of the total pressure. On the other hand, if the entropy profiles are not flat at the outskirts as indicated by combined Plank and ROSAT observations, the carried energy and the CR pressure should be much smaller than the above estimations.