In a growing number of galaxy clusters diffuse elongated radio sources have been found, so-called radio relics. These megaparsec relics are usually located in the low-density outskirts of merging clusters. The existence of these radio sources implies the presence of relativistic particles and magnetic fields in the intracluster medium. The idea is that these radio relics trace cluster merger shocks, where particles are accelerated to relativistic energies, causing them to emit synchrotron radiation observable at radio wavelengths. However, a problem is that cluster merger shocks should have low-Mach numbers, for which particle acceleration is thought to be very inefficient. This is hard to reconcile with the observed synchrotron radiation. Current X-ray observations lack the sensitivity to determine accurate shock properties, since these relics are located in the faint outskirts of clusters. Observations with the X-Ray Surveyor will for the first time provide accurate measurements of the Mach number of these shocks, both using the temperature and surface brightness. This will provide crucial information to determine the nature of the particle acceleration mechanism that operates this intracluster plasma.

Session 2: Plasma Physics Chair: Steve Allen