  Radio relics are Mpc-scale diffuse synchrotron sources found in galaxy
cluster outskirts. They are believed to be associated with large-scale shocks
propagating through the intra-cluster medium, although the connection between
radio relics and the cluster merger shocks is not yet proven conclusively. We
present a first tentative detection of a pressure jump in the well-known relic
of the Coma cluster through Sunyaev-Zel'dovich (SZ) effect imaging.The SZE data
are extracted from the first public all-sky data release of Planck and we use
high-frequency radio data at 2.3 GHz to constrain the shock-front geometry. The
SZE data provide evidence for a pressure discontinuity, consistent with the
relic position, without requiring any additional prior on the shock-front
location. The derived Mach number M = 2.9 (+0.8/-0.6) is consistent with X-ray
and radio results. A high-pressure "filament" without any pressure
discontinuity is disfavoured by X-ray measurements and a "sub-cluster" model
based on the infalling group NGC 4839 can be ruled out considering the
published mass estimates for this group. These results signify a first attempt
towards directly measuring the pressure discontinuity for a radio relic and the
first SZ-detected shock feature observed near the virial radius of a galaxy
cluster.

Cosmology and Nongalactic Astrophysics