Merger Shocks in Abell 3667 and the Cygnus~A Cluster
  We present new XMM-Newton observations of the northwest (NW) radio relic
region in the cluster Abell 3667. We detect a jump in the X-ray surface
brightness and X-ray temperature at the sharp outer edge of the radio relic
which indicate that this is the location of a merger shock with a Mach number
of about 2. Comparing the radio emission to the shock properties implies that
approximately 0.2% of the dissipated shock kinetic energy goes into
accelerating relativistic electrons. This is an order of magnitude smaller than
the efficiency of shock acceleration in many Galactic supernova remnants, which
may be due to the lower Mach numbers of cluster merger shocks. The X-ray and
radio properties indicate that the magnetic field strength in the radio relic
is >= 3 muG, which is a very large field at a projected distance of ~2.2 Mpc
from the center of a cluster. The radio spectrum is relatively flat at the
shock, and steepens dramatically with distance behind the shock. This is
consistent with radiative losses by the electrons and the post-shock speed
determined from the X-ray properties. The Cygnus A radio source is located in a
merging cluster of galaxies. This appears to be an early-stage merger. Our
recent Suzaku observation confirm the presence of a hot region between the two
subclusters which agrees with the predicted shocked region. The high spectral
resolution of the CCDs on Suzaku allowed us to measure the radial component of
the merger velocity, Delta v_r \approx 2650 km/s.
