High and ultra-high energy cosmic ray acceleration in   structure-formation shocks in Abell 3376 galaxy cluster
  We describe multiwavelength optical, X-ray and radio evidences of energetic mergers in southern galaxy cluster Abell 3376 (A3376, redshift z=0.046, X-ray luminosity $ \rm L_{X}(0.1-2.4 keV)= 2.48 \times 10^{44} erg s^{-1}$). Here we report the rare discovery of gigantic, Mpc-scale non-thermal radio structures (`radio-arcs'), shaped like a pair of bow-shock fronts on the merger axis. We argue for the shock acceleration origin of the radio emission and discuss the possibilty of acceleration of very high energy cosmic ray particles in this and similar clusters. At redshift z=0.04, cluster A3376 is near enough that some of these primary or secondary cosmic ray components could actually be detected by new cosmic ray telescopes, particularly the energetic inverse Compton hard X-ray/gamma-ray photons upto extreme energies $E_{\gamma} \sim 100 {\gamma^{e}_{7}}^{2}$ GeV (where ${\gamma^{e}_{7}}$ is the electron Lorentz factor in units of $10^7$). Identification of radio or hard X-ray/gamma-ray emission from structure formation shocks holds a great promise for advancing current knowledge on shock formation in the IGM. It should provide the first direct evidence for such shocks, revealing the underlying large-scale cosmological flows. When combined with hard X-ray or gamma-ray detection, the radio signal will provide a direct measure of the unknown magnetic fields in the IGM, thus providing a better understanding of processes leading to IGM magnetization and also acceleration and propagation of ultra-high energy cosmic rays. 
J. Bagchi (IUCAA, Pune, India), Florence Durret (IAP, Paris), Gastao   B. Lima Neto (IAG, Sao Paulo, Brazil), Surajit Paul (...