Shock acceleration as origin of the radio relic in A521?

TL;DR

This study uses multi-frequency radio and X-ray observations to analyze the radio relic in A521, providing evidence that shock acceleration of electrons is the origin of the relic, supported by spectral analysis and shock detection.

Contribution

The paper offers new high-sensitivity radio observations and spectral analysis of the A521 relic, linking it to shock acceleration and identifying a potential shock front via X-ray data.

Findings

Radio spectrum is a single power law.

Spectral index steepens from outer to inner relic.

X-ray data shows a shock front coincident with the relic.

Abstract

We present new high sensitivity observations of the radio relic in A521 carried out with the Giant Metrewave Radio Telescope at 327 MHz and with the Very Large Array at 4.9 and 8.5 GHz. We imaged the relic at these frequencies and carried out a detailed spectral analysis, based on the integrated radio spectrum between 235 MHz and 4.9 GHz, and on the spectral index image in the frequency range 327-610 MHz. To this aim we used the new GMRT observations and other proprietary as well as archival data. We also searched for a possible shock front co-located with the relic on a short archival Chandra X-ray observation of the cluster. The integrated spectrum of the relic is consistent with a single power law; the spectral index image shows a clear trend of steepening going from the outer portion of the relic toward the cluster centre. We discuss the origin of the source in the light of the theoretical models for the formation of cluster radio relics. Our results on the spectral properties of the relic are consistent with acceleration of relativistic electrons by a shock in the intracluster medium. This scenario is further supported by our finding of an X-ray surface brightness edge coincident with the outer border of the radio relic. This edge is likely a shock front.