Abell 2256 - Observing a Mpc(3) nonthermal laboratory

The galaxy cluster Abell 2256 is in the violent stage of a giant cluster merger event. Evidence of this merger is seen in the X-ray substructure of ROSAT images (Briel et al. 1991). Radio images reveal the complex nature of this cluster: a synchrotron halo, several head-tail radio galaxies, and two extended irregular and sharp-edged regions of diffuse radio emission - so called radio relics (Fig. 1). The relics are believed to be powered by the energy input of the merger shock into an old (relic) relativistic electron population. This theory is supported by the observed radio polarization properties of the relics which closely match the polarization predicted by Ensslin et al. (1998) to result from synchrotron emission in shock-compressed magnetic fields. We report on an on-going project designed to obtain detailed Faraday rotation measure maps of these radio relics. The differential "Faraday screen" effect over extended radio sources allows us to study the magnetic field distribution inside these sources and in the foreground Faraday rotating medium