Population diagrams extracted from the positions shown in Fig. 3 and corresponding to the chi-square test shown in Fig. 8. The black data points represent the N_up measurements and their uncertainties. The red circles represent the best-fit non-LTE model obtained for the S(0) to S(7) transitions. The corresponding power-law indices and total column densities are shown in the bottom-left corner of the diagrams. The results of the χ² test (see text) are presented in the top-right corner of the diagrams. The order of the diagrams (from left to right and top to bottom) is the same as the order of the white circles in Fig. 3.