  When galaxy clusters collide, they generate shock fronts in the hot
intracluster medium. Observations of these shocks can provide valuable
information on the merger dynamics and physical conditions in the cluster
plasma, and even help constrain the nature of dark matter. To study shock
fronts, one needs an X-ray telescope with high angular resolution (such as
Chandra), and be lucky to see the merger from the right angle and at the right
moment. As of this writing, only a handful of merger shock fronts have been
discovered and confirmed using both X-ray imaging and gas temperature data --
those in 1E0657-56, A520, A754, and two fronts in A2146. A few more are
probable shocks awaiting temperature profile confirmation -- those in A521,
RXJ1314-25, A3667, A2744, and Coma. The highest Mach number is 3 in 1E0657-56,
while the rest has M=1.6-2. Interestingly, all these relatively weak X-ray
shocks coincide with sharp edges in their host cluster's synchrotron radio
halos (except in A3667, where it coincides with the distinct radio relic, and
A2146, which does not have radio data yet). This is contrary to the common
wisdom that weak shocks are inefficient particle accelerators, and may shed
light on the mechanisms of relativistic electron production in astrophysical
plasmas.
