Magnetic Fields, Relativistic Particles, and Shock Waves in Cluster Outskirts

TL;DR

Recent advances in multi-wavelength observations have significantly improved understanding of shock waves, magnetic fields, and relativistic particles in the outskirts of galaxy clusters, revealing new insights into plasma physics in these extreme environments.

Contribution

This review synthesizes recent observational and theoretical progress on magnetic fields, relativistic particles, and shocks in galaxy cluster outskirts, highlighting new evidence and understanding.

Findings

Detection of shock fronts via X-ray observations.

Estimates of magnetic field strengths up to the virial radius.

Evidence for particle acceleration at shock fronts from radio relics.

Abstract

It is only now, with low-frequency radio telescopes, long exposures with high-resolution X-ray satellites and gamma-ray telescopes, that we are beginning to learn about the physics in the periphery of galaxy clusters. In the coming years, Sunyaev-Zeldovich telescopes are going to deliver further great insights into the plasma physics of these special regions in the Universe. The last years have already shown tremendous progress with detections of shocks, estimates of magnetic field strengths and constraints on the particle acceleration efficiency. X-ray observations have revealed shock fronts in cluster outskirts which have allowed inferences about the microphysical structure of shocks fronts in such extreme environments. The best indications for magnetic fields and relativistic particles in cluster outskirts come from observations of so-called radio relics, which are megaparsec-sized regions of radio emission from the edges of galaxy clusters. As these are difficult to detect due to their low surface brightness, only few of these objects are known. But they have provided unprecedented evidence for the acceleration of relativistic particles at shock fronts and the existence of muG strength fields as far out as the virial radius of clusters. In this review we summarise the observational and theoretical state of our knowledge of magnetic fields, relativistic particles and shocks in cluster outskirts.