Shocking news outside of cluster cores
  With magnetic fields below one microgauss, the minimum pressures of synchrotron plasmas start to approach those in the thermal gas in cluster outskirts and the more diffuse WHIM in large-scale structure filaments. We summarize some of our techniques to find the corresponding low surface brightness radio sources and what we have uncovered. We identify cluster-like sources in poor environments, likely requiring very efficient relativistic particle acceleration, as well as very diffuse radio galaxies with no indications of current activity. More detailed observations of Coma and a new radio-identified cluster highlight some emerging issues in cluster-related shocks: Coma has a shock on the border of its halo, suggesting a connection between the two; the Coma relic appears to be an "infall" shock, associated with a 500 km/s infalling column of galaxies; the new cluster has filamentary pieces in addition to its more classical halo and peripheral relics, suggesting that we are beginning to see relic emission face on. Finally, a different kind of shock comes from our recent finding of ambiguities in the determination of rotation measures which will influence cluster magnetic field studies. 
Lawrence Rudnick, Shea Brown, Damon Farnsworth
astro-ph.CO