Discovery of giant `radio arcs' in cluster Abell 3376: evidence for   shock acceleration in a violent cluster merger?
  New multi-wavelength (radio, optical & X-rays) observational evidences are presented which show that the nearby (z=0.046), rich cluster of galaxies Abell 3376 is experiencing a major event of binary subcluster merger. The key evidence is the discovery of a pair of large, optically unidentified diffuse radio sources (`arcs'), symmetrically located about 2.6 h_{50}^{-1} Mpc apart at the opposite ends of the hot intra-cluster gas mapped by ROSAT in X-rays. It is argued that the gas-dynamical shock-waves, which occur naturally during cluster formation, are accelerating charged particles (cosmic rays) to relativistic energies, leading to synchrotron emission from the megaparsec scale radio arcs. If this is so, cluster Abell 3376 would also be a potential source capable of accelerating cosmic ray particles upto ultra-high energies (UHECR) of E_{max} ~ 10^{18-19} eV. Thus this cluster is an excellent test-bed for understanding the physics of merger shocks and origin of enigmatic UHECR particles in structure formation process. Hence, Abell 3376 provides unique oppurtunities for further multi-wavelength observations with ground and space-borne observatories. 
Joydeep Bagchi