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