Upper panel: best-fit values of N_H for each Obsid as a function of the Mission Timeline MET (years). The first point has been moved to 21.5 to be shown in the same linear plot. The horizontal shaded line shows the best-fit value of the cumulative spectrum with PSF correction, as listed in Table 1. Lower panel: same, but with best-fit values of N_H obtained with Γ frozen to 1.88. The vertical dashed lines bracket the period with low intrinsic absorption as discussed in the text.