Constraining the efficiency of cosmic ray acceleration by cluster shocks

TL;DR

This study uses cosmological simulations and Fermi satellite data to constrain the efficiency of cosmic ray acceleration by cluster shocks, finding it to be lower than previously assumed, which challenges existing models.

Contribution

The paper provides new constraints on shock acceleration efficiency in galaxy clusters using advanced simulations and gamma-ray observations, indicating a need to revise current theories.

Findings

Acceleration efficiency is less than 0.1% for galaxy cluster shocks.

Results are consistent across non-radiative and radiative simulations including AGN feedback.

Current models overestimate the shock acceleration efficiency in the intracluster medium.

Abstract

We study the acceleration of cosmic rays by collisionless structure formation shocks with ENZO grid simulations. Data from the FERMI satellite enable the use of galaxy clusters as a testbed for particle acceleration models. Based on advanced cosmological simulations that include different prescriptions for gas and cosmic rays physics, we use the predicted {\gamma}-ray emission to constrain the shock acceleration efficiency. We infer that the efficiency must be on average <0.1% for cosmic shocks, particularly for the 2<M<5 merger shocks that are mostly responsible for the thermalisation of the intracluster medium. These results emerge, both, from non-radiative and radiative runs including feedback from active galactic nuclei, as well as from zoomed resimulations of a cluster resembling MACSJ1752.0+0440. The limit on the acceleration efficiency we report is lower than what has been assumed in the literature so far. Combined with the information from radio emission in clusters, it appears that a revision of the present understanding of shock acceleration in the ICM is unavoidable.