SPECTRAL INDEX STUDIES OF THE DIFFUSE RADIO EMISSION IN ABELL 2256: IMPLICATIONS FOR MERGER ACTIVITY

We present a multi-wavelength analysis of the merging rich cluster of galaxies, Abell 2256 (A2256). We have observed A2256 at 150MHz using the Giant Metrewave Radio Telescope and successfully detected the diffuse radio halo and the relic emission over a similar to 1.2 Mpc(2) extent. Using this 150MHz image and the images made using archival observations from the Very Large Array (VLA; 1369 MHz) and the Westerbrok Synthesis Radio Telescope (WSRT; 330 MHz), we have produced spectral index images of the diffuse radio emission in A2256. These spectral index images show a distribution of flat spectral index (S alpha v(alpha), alpha in the range -0.7 to -0.9) plasma in the region NW of the cluster center. Regions showing steep spectral indices (alpha in the range -1.0 to -2.3) are toward the SE of the cluster center. These spectral indices indicate synchrotron lifetimes for the relativistic plasmas in the range 0.08-0.4 Gyr. We interpret this spectral behavior as resulting from a merger event along the direction SE to NW within the last 0.5 Gyr or so. A shock may be responsible for the NW relic in A2256 and the megaparsec scale radio halo toward the SE is likely to be generated by the turbulence injected by mergers. Furthermore, the diffuse radio emission shows spectral steepening toward lower frequencies. This low-frequency spectral steepening is consistent with a combination of spectra from two populations of relativistic electrons created at two epochs (two mergers) within the last similar to 0.5 Gyr. Earlier interpretations of the X-ray and the optical data also suggested that there were two mergers in Abell 2256 in the last 0.5 Gyr, consistent with the current findings. Also highlighted in this study is the futility of correlating the average temperatures of thermal gas and the average spectral indices of diffuse radio emission in the respective clusters.