XMM-Newton Observation of the Northwest Radio Relic Region in Abell 3667

TL;DR

This study uses XMM-Newton observations to analyze the NW radio relic in Abell 3667, revealing temperature and brightness changes consistent with a shock, and constraining the magnetic field and particle acceleration mechanisms.

Contribution

First detailed X-ray analysis of the NW relic in Abell 3667 showing shock properties and magnetic field estimates using XMM-Newton data.

Findings

Temperature declines from 6 to 1 keV across the relic.

Evidence of a shock with Mach number ~2 and shock speed ~1200 km/s.

Lower limit of magnetic field strength in the relic is 3 microGauss.

Abstract

Abell 3667 is the archetype of a merging cluster with radio relics. The 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 40 ksec of net XMM 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~2 shock and a shock speed of ~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 ~0.2%. Comparing the limit on the inverse Compton X-ray emission with the measured radio synchrotron emission, we set a lower limit to the magnetic field in the relic of 3 muG. If the emission from the relic is non-thermal, this lower limit is in fact the required magnetic field.