Left panel: hardness ratio (see Sect. 6.4) vs. the 2–10 keV flux, for the H HGL sample. The red triangles represent lower limits hardness ratios evaluated using 50% S count rate upper limits. The crosses are the mean hardness ratios in flux bins, evaluated using the “survival analysis” (Kaplan & Meier 1958; Miller 1981, p. 74). The mean 1σ errors in the hardness ratio at different fluxes are indicated by the error bars plotted at the bottom. The dotted and dashed lines show for comparison the hardness ratio for an absorbed power-law model of photon index Γ = 1.8 and column density 1022 cm-2 and 1023 cm-2, respectively, at redshift decreasing from 2 to 0 (from bottom to top). Right panel: hardness ratio (see Sect. 6.4) vs. the 0.5–2 keV flux, for the S HGL sample, here the red triangles are upper limits hardness ratios evaluated using 50% H count rate upper limits. Other symbols like in right panel.
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