XMM-Newton Observations of the Southeastern Radio Relic in Abell 3667
  Radio relics, elongated, non-thermal, structures located at the edges of
galaxy clusters, are the result of synchrotron radiation from cosmic-ray
electrons accelerated by merger-driven shocks at the cluster outskirts.
However, X-ray observations of such shocks in some clusters suggest that they
are too weak to efficiently accelerate electrons via diffusive shock
acceleration to energies required to produce the observed radio power. We
examine this issue in the merging galaxy cluster Abell 3667 (A3667), which
hosts a pair of radio relics. While the Northwest relic in A3667 has been well
studied in the radio and X-ray by multiple instruments, the Southeast relic
region has only been observed so far by Suzaku, which detected a temperature
jump across the relic, suggesting the presence of a weak shock. We present
observations of the Southeastern region of A3667 with XMM-Newton centered on
the radio relic. We confirm the existence of an X-ray shock with Mach number of
about 1.8 from a clear detection of temperature jump and a tentative detection
of a density jump, consistent with previous measurements by Suzaku. We discuss
the implications of this measurement for diffusive shock acceleration as the
main mechanism for explaining the origin of radio relics. We then speculate on
the plausibility of alternative scenarios, including re-acceleration and
variations in the Mach number along shock fronts.
