088071

Among over thousand X-ray luminous galaxy clusters found in our systematic cluster survey comma A1631 is the most extreme comma diffuse system comma with a number of peculiar properties. It has a very low surface brightness comma one of the lowest central ICM densities comma and the highest known central entropy. The galaxy distribution shows a surprisingly low concentration parameter and shallow galaxy distribution. Its X-ray luminosity is much lower than the estimate from the Lx-mass relation using the dynamical mass. A first explanation comma that this system is in formation comma seems not consistent with its high central entropy. It is highly important to understand this extreme and puzzling system at the edge of the morphological distribution through a detailed spectro-imaging study with XMM-Newton.