XMM-Newton ICM X-ray contours of constant X-ray surface brightness, superimposed onto DSS2-r band images of the parent galaxy cluster. The green circles represent extended sources and the green squares refer to point sources. The eight clusters are grouped in pairs that belong to four increasing intervals of cosmological redshifts (z ∼ 0.15; 0.4 < z < 0.7; 1 < z < 1.1; 1.3 < z < 1.5).