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The results of long XMM-Newton X-ray observations of the NW radio relic ofAbell 3667 are presented. A shock is detected at the sharp outer edge of theradio relic, both in the X-ray surface brightness and the temperature profiles.The Mach number is M = 2.54^+0.80_-0.43. The temperature jump at the shock islarger than expected from the density jump, which may indicate that adynamically important magnetic field aligned primarily parallel to the shockfront is present. The gas temperature rises gradually over several arc minuteswithin the shock region. This could indicate that the shock energy is initiallydissipated into some mix of thermal and nonthermal (e.g., turbulence)components, and that the nonthermal energy decays into heat in the post-shockregion. The observed radio relic can be powered if ~0.2% of the energydissipated in the shock goes into the (re)acceleration of relativisticelectrons. We show that the observed steepening of the radio spectrum withdistance behind the shock is consistent with radiative losses by theradio-emitting electrons. However, the radio spectrum immediately behind theshock is flatter than expected for linear diffusive shock acceleration ofthermal electrons. This suggests that the shock re-accelerates a pre-existingpopulation of relativistic electrons. We also detect a bright, cool region (the"Mushroom") to the south of the radio relic, which we propose is the remnantcool core of a merging subcluster, and that this subcluster was the driver forthe observed NW shock. In this model, the properties of Abell 3667 are mainlythe result of an offset binary merger, and the cluster is being observed about1 Gyr after first core passage. We predict that deeper X-ray or SZ observationsof the SE radio relic will reveal a second merger shock at the outer edge.

Deep XMM-Newton Observations of the NW Radio Relic Region of Abell 3667