Hardness ratio diagram. Solid lines represent the expected hardness ratio locus for power‐law X‐ray spectra; dashed lines mark thermal plasma spectra. The tracks have as a free parameter the X‐ray–absorbing column density. Starting leftmost with the unabsorbed situation, ﹩N_{\mathrm{H}\,\,{}^{{\rm{\small I}}}}=0,\,2,\,3,﹩ and ﹩4\times 10^{20}\,\mathrm{cm}\,^{-2}﹩. The numbers along the ﹩N_{\mathrm{H}\,\,{}^{{\rm{\small I}}}}=3\times 10^{20}\,\mathrm{cm}\,^{-2}﹩ track give the energy index in case of power law and ﹩\mathrm{log}\,T( \mathrm{K}\,) ﹩ in the case of thermal plasma spectra. To illustrate the case of very high column densities, we also plotted the color‐color tracks for ﹩N_{\mathrm{H}\,\,{}^{{\rm{\small I}}}}=10\times 10^{20}\,\mathrm{cm}\,^{-2}﹩, both for thermal plasma emission and for power‐law X‐ray spectra.