Particle Acceleration on Megaparsec Scales in a Merging Galaxy Cluster

TL;DR

This paper reports the detection of a radio relic in a merging galaxy cluster, demonstrating that diffusive shock acceleration operates on megaparsec scales and can produce highly energetic cosmic rays.

Contribution

It provides observational evidence that diffusive shock acceleration occurs on large scales in galaxy clusters, expanding understanding of cosmic ray origins.

Findings

Detection of a radio relic with aligned magnetic fields

Observation of a strong spectral index gradient

Measurement of a narrow relic width indicating magnetic field strength

Abstract

Galaxy clusters form through a sequence of mergers of smaller galaxy clusters and groups. Models of diffusive shock acceleration (DSA) suggest that in shocks that occur during cluster mergers, particles are accelerated to relativistic energies, similar to supernova remnants. Together with magnetic fields these particles emit synchrotron radiation and may form so-called radio relics. Here we report the detection of a radio relic for which we find highly aligned magnetic fields, a strong spectral index gradient, and a narrow relic width, giving a measure of the magnetic field in an unexplored site of the universe. Our observations prove that DSA also operates on scales much larger than in supernova remnants and that shocks in galaxy clusters are capable of producing extremely energetic cosmic rays.