First X-ray evidence for a shock at the Coma relic

TL;DR

This study provides the first X-ray evidence of a shock at the Coma relic, supporting the infall shock hypothesis over previous turbulence or outgoing merger shock scenarios.

Contribution

It presents the first X-ray detection of a shock at the Coma relic, clarifying its origin as an infall shock from the NGC 4839 group.

Findings

Detected a shock with Mach number about 2 at the relic

Excludes turbulence as the formation mechanism for the relic

Supports the infall shock scenario from the NGC 4839 group

Abstract

The Coma cluster is one of the nearest galaxy clusters, and the first one in which a radio halo and a peripheral relic were discovered. While its halo and the central parts of the intracluster medium have been studied extensively, X-ray observations of the plasma near its relic have been scarce. Here, we present results from a re-analysis of a 22-ks archival XMM-Newton observation. Across the relic, we detect a shock of Mach number about 2. This excludes the previously suggested hypothesis that the relic was formed by turbulence. Furthermore, multiwavelenth observations and numerical models do not support the scenario in which the shock at the Coma relic is an outgoing cluster-merger shock. Instead, our results lend support to the idea that the relic coincides with an infall shock front formed just as the NGC 4839 group falls onto the cluster along a cosmic filament.