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 clusters 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.
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