XMM-Newton observations of Abell 2255 : a test case of a merger after   `core-crossing'
  It has been known that Abell 2255 is not a relaxed cluster, but it is undergoing a merger. Here, we report on the analysis of the XMM-Newton observations of this cluster. The X-ray data give us the opportunity to reveal the complexity of the cluster, especially its temperature distribution. The integrated spectrum is well fitted by a single temperature thermal model, indicating a mean temperature of ~7 keV. However, the cluster is not isothermal at this temperature: its eastern regions are significantly cooler, at ~5.5 keV, whilst towards the West the temperature reaches ~8.5 keV.   These temperature asymmetries can be explained if Abell 2255 has been assembled recently by the merging of smaller subunits. It is now in the phase after the cores of these subunits have collided (the `core-crossing' phase) some 0.1-0.2 Gyr ago. A comparison with numerical simulations suggests that it will settle down into a single relaxed cluster in ~(2-3) Gyr. 
Irini Sakelliou, Trevor J. Ponman