Expected hardness ratio (of the count rates in the energy ranges 1–3 to 3–10 keV) as a function of the column density. Solid and dashed lines refer to the pn and the MOS cameras, respectively. Red, green, and blue lines refer to power-law spectra with photon indices of 2, 1.5, and 1, respectively. The horizontal line at HR = 0.7 represents the threshold value used to select highly absorbed sources.