ChandraObservation of A2256: A Cluster at the Early Stage of Merging

We present here \chandra observations of the rich cluster of galaxies A2256.In addition to the known cool subcluster, a new structure was resolved 2$'$east of the peak of the main cluster. Its position is roughtly at the center ofa low-brightness radio halo. Spectral analysis shows that the "shoulder" hashigh iron abundance ($\sim$ 1). We suggest that this structure is eitheranother merging component or an internal structure of the main cluster. TheX-ray redshifts of several regions were measured. The results agree with theoptical ones and suggest that the main cluster, the subcluster and the"shoulder" are physically associated and interacting. The subcluster has lowtemperature ($\sim$ 4.5 keV) and high iron abundance ($\sim$ 0.6) in thecentral 150 kpc. The \chandra image shows a relatively sharp brightnessgradient at the south of the subcluster peak running south-south-east (SSE). Atemperature jump was found across the edge, with higher temperature ahead ofthe edge in the low density region. This phenomenon is qualitatively similar tothe "cold fronts" found in A2142 and A3667. If the "shoulder" is ignored, thetemperature map resembles those simulations at the early stage of merging whilethe subcluster approached the main cluster from somewhere west. This fact andthe observed edge, in combination with the clear iron abundance contrastbetween the center of the subcluster ($\sim$ 0.6) and the main cluster ($\sim$0.2), all imply that the ongoing merger is still at the early stage. At leastthree member galaxies, including a radio head-tail galaxy, were found to havecorresponding X-ray emission.