Hardness ratios modeled for XRT as a function of redshift for groups and clusters with several temperatures (red continuous lines, kT respectively equal to 0.5, 1.0, 2.0, 3.0, 5.0, 7.0, 10.0 and 12.0 keV from the bottom to the top) and for AGN with different intrinsic absorption (blue dashed lines, N_H respectively equal to 0.01, 0.1, 0.3, 1.0, 3.0 and 10.0  ×  10²² cm^-2 from the bottom to the top). In the top right, we show the typical error bar on HR for a source with 200 total net counts.