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 \sim 7 keV .
However , the cluster is not isothermal at this temperature : its eastern regions are significantly cooler , at \sim 5.5 keV , whilst towards the West the temperature reaches \sim 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 \sim ( 2-3 ) Gyr .