On the Challenges of Cosmic-Ray Proton Shock Acceleration in the Intracluster Medium

TL;DR

This paper reviews the latest advances in understanding why cosmic-ray protons in galaxy clusters' intracluster medium are not detected via gamma-ray emissions, challenging existing shock acceleration models.

Contribution

It provides a comprehensive summary of recent theoretical and numerical developments addressing the missing gamma-ray problem in galaxy clusters.

Findings

Models predict gamma-ray signals below current detection limits.

Microphysics of shock acceleration is better understood.

The missing gamma-ray problem remains unresolved.

Abstract

Galaxy clusters host the largest particle accelerators in the Universe: Shock waves in the intracluster medium (ICM), a hot and ionised plasma, that accelerate particles to high energies. Radio observations pick up synchrotron emission in the ICM, proving the existence of accelerated cosmic-ray electrons. However, a sign of cosmic-ray protons, in form of $\gamma$-rays. remains undetected. This is know as the missing $\gamma$-ray problem and it directly challenges the shock acceleration mechanism at work in the ICM. Over the last decade, theoretical and numerical studies focused on improving our knowledge on the microphysics that govern the shock acceleration process in the ICM. These new models are able to predict a $\gamma$-ray signal, produced by shock accelerated cosmic-ray protons, below the detection limits set modern $\gamma$-ray observatories. In this review, we summarise the latest advances in solving the missing $\gamma$-ray problem.