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

Abstract

We present a multi-wavelength analysis of the merging rich cluster of galaxies Abell 2256. We have observed A2256 at 150 MHz using the Giant Metrewave Radio Telescope and successfully detected the diffuse radio halo and the relic emission over an extent $\sim1.2$ Mpc$^2$. Using this 150 MHz image and the images made using archival observations from the VLA (1369 MHz) and the WSRT (350 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$\propto\nu^\alpha$, $\alpha$ in the range -0.7 to -0.9) plasma in the NW of the cluster centre. Regions showing steep spectral indices ($\alpha$ in the range -1.0 to -2.3) are toward the SE of the cluster centre. These spectral indices indicate synchrotron life times for the relativistic plasmas in the range 0.08 - 0.4 Gyr. We interpret this spectral behaviour 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 Mpc scale radio halo towards 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 $\sim$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 respective clusters.