Low flux end of the differential number counts in the 0.5−2 keV band normalized to the Euclidean slope. The data points, the dotted and dot-dash curves as in Fig. 10. Broken solid lines: the NNST solutions for the N(S) with the bright end normalization according to Georgakakis et al. (2008) – label “1.00”, and the normalization reduced by 20% – label “0.80”. The label “HM” indicates the best estimate of counts obtained by Hickox & Markevitch (2007b).