Benchmark Study of the Most Extreme comma Diffuse Galaxy Cluster Abell 1631

Among over thousand Xray luminous galaxy clusters found in our systematiccluster survey, A1631 is the most extreme, diffuse system, with a number ofpeculiar properties. It has a very low surface brightness, one of the lowestcentral ICM densities, and the highest known central entropy. The galaxydistribution shows a surprisingly low concentration parameter and shallow galaxydistribution. Its Xray luminosity is much lower than the estimate from theLxmass relation using the dynamical mass. A first explanation, that this systemis in formation, seems not consistent with its high central entropy. It ishighly important to understand this extreme and puzzling system at the edge ofthe morphological distribution through a detailed spectroimaging study with XMMNewton. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]