Xmm-Newton Observation Of The Northwest Radio Relic Region In A3667

A3667 is the archetype of a merging cluster with radio relics. The northwest (NW) radio relic is the brightest cluster relic or halo known and is believed to be due to a strong merger shock. We have observed the NW relic for similar to 40 ks of net XMM-Newton time. We observe a global decline of temperature across the relic from 6 to 1 keV, similar to the Suzaku results. Our new observations reveal a sharp change of both temperature and surface brightness near the position of the relic. The increased X-ray emission on the relic can be equivalently well described by either a thermal or nonthermal spectral model. The parameters of the thermal model are consistent with a Mach number M similar to 2 shock and a shock speed of similar to 1200 km s(-1). The energy content of the relativistic particles in the radio relic can be explained if they are (re)-accelerated by the shock with an efficiency of similar to 0.2%. Comparing the limit on the inverse Compton X-ray emission with themeasured radio synchrotron emission, we set a lower limit to themagnetic field in the relic of 3 mu G. If the emission from the relic is nonthermal, this lower limit is in fact the required magnetic field.