Entropy at the Outskirts of Galaxy Clusters as Implications for Cosmological 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.

Accepted for publication in ApJ Letters - typos corrected, references updated

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• The Entire Virial Radius of the Fossil Cluster RXJ1159+5531: I. Gas Properties
• Collisionless shocks in partly ionized plasma with cosmic rays: microphysics of non-thermal components
• Suzaku X-ray Observations of the Nearest Non-Cool Core Cluster, Antlia: Dynamically Young but with Remarkably Relaxed Outskirts
• Probing WHIM around Galaxy Clusters with Fast Radio Bursts and the Sunyaev-Zel'dovich effect
• Multiwavelength study of X-ray Luminous Clusters in the Hyper Suprime-Cam Subaru Strategic Program S16A field
• Nonthermal Pressure in the Outskirts of Abell 2142
• Excess entropy and energy feedback from within cluster cores up to r${}_{200}$
• Optical inverse-Compton emission from clusters of galaxies
• SuperModel Analysis of Abell 1246 and J255: on the Evolution of Galaxy Clusters from High to Low Entropy States
• Particle Reacceleration by Turbulence and Radio Constraints on Multi-Messenger High-Energy Emission from the Coma Cluster