Discovery of Gas Bulk Motion in the Galaxy Cluster Abell 2256 with Suzaku

The results from Suzaku observations of the galaxy cluster Abell 2256 are presented. This cluster is a prototypical and well-studied merging system, exhibiting substructures both in the X-ray surface brightness and in the radial velocity distribution of member galaxies. There are main and sub components separated by 3'.5 in the sky and by about 2000 km s(-1) in radial-velocity peaks of the member galaxies. In order to measure the Doppler shifts of iron K-shell lines from the two gas components by the Suzaku XIS, the energy scale of the instrument was carefully evaluated and found to be calibrated well. A significant shift of the radial velocity of the sub component gas with respect to that of the main cluster was detected. All three XIS sensors show the shift independently and consistently among the three. The difference is found to be 1500 +/- 300 (statistical) +/- 300 (systematic) km s(-1). The X-ray determined absolute redshifts of, and hence the difference between, the main and sub components are consistent with those of member galaxies in the optical band. The observation indicates robustly that the X-ray emitting gas is moving together with galaxies as a substructure within the cluster. These results along with other X-ray observations of gas bulk motions in merging clusters are discussed.