Solid lines: column density evolution as derived from model calculations. The lines are presented for several values (cm⁻²) listed in the legend. Additionally we plotted locations of HXR sources (diamonds and dashed lines). The colour-coding relates energies with relevant column densities. Colours are the same as in Fig. 6, the red line is for 55 − 75 keV, yellow presents 21 − 25 keV, etc.