Estimated upper limits (ΔC = 3.84, equivalent to the 95% confidence level in the Gaussian approximation) on the column density N_H(z) as a function of the observed hardness ratio for each source of our sample. Such constraints are obtained from the spectral fits with an absorbed power-law model, where an intrinsic continuum with Γ = 1.8 was adopted for the faintest objects (red symbols). The synthetic N_H(z)–HR curves corresponding to an absorbed spectrum with Γ = 2.0 (black) and 1.5 (magenta) are also plotted for reference. These have been computed for the mean Galactic column ( cm⁻²) and quasar redshift (z = 3.14), assuming the energy-dependent response of the pn detector. The grey and lilac shaded areas respectively illustrate the uncertainties due to the full range of and z covered by the sample. The large majority of our objects are consistent with being, at most, mildly obscured (i.e. N_H < a few ×10²² cm⁻²).