摘要：Radio relics in galaxy clusters are believed to be associated with powerful shock fronts
that originate during cluster mergers, and are a testbed for the acceleration of
relativistic particles in the intracluster medium. Recently, radio relic observations have
pushed into the cm-wavelength domain (1–30 GHz) where a break from the standard
synchrotron power law spectrum has been found, most noticeably in the famous “Sausage”
relic. Such spectral steepening is seen as an evidence for non-standard relic models, such
as ones requiring seed electron population with a break in their energy spectrum. In this
paper, however, we point to an important effect that has been ignored or considered
insignificant while interpreting these new high-frequency radio data, namely the
contamination due to the Sunyaev-Zel’dovich (SZ) effect that changes the observed
synchrotron flux. Even though the radio relics reside in the cluster outskirts, the
shock-driven pressure boost increases the SZ signal locally by roughly an order of
magnitude. The resulting flux contamination for some well-known relics are non-negligible
already at 10 GHz, and at 30 GHz the observed synchrotron fluxes can be diminished by a
factor of several from their true values. At higher redshift the contamination gets
stronger due to the redshift independence of the SZ effect. Interferometric observations
are not immune to this contamination, since the change in the SZ signal occurs roughly at
the same length scale as the synchrotron emission, although there the flux loss is less
severe than single-dish observations. Besides presenting this warning to observers, we
suggest that the negative contribution from the SZ effect can be regarded as one of the
best evidence for the physical association between radio relics and shock waves. We
present a simple analytical approximation for the synchrotron-to-SZ flux ratio, based on a
theoretical radio relic model that connects the nonthermal emission to the thermal gas
properties, and show that by measuring this ratio one can potentially estimate the relic
magnetic fields or the particle acceleration efficiency.
