Comparison of the 2-keV monochromatic flux estimate derived from photometric data (see L20) and the one derived from spectroscopic data for the subsample of objects with only one observation in the 4XMM-DR9 catalogue. The solid red line represents the bisector, while the black dashed line is the best fit, with a slope of η = 1.06 ± 0.02. The total scatter around the best fit is equal to the average uncertainties, δ_err = δ_tot = 0.14 dex. The intrinsic dispersion of the relation is therefore zero. Overall, we can say that there is no significant contribution to the observed dispersion that comes from using photometric X-ray data instead of spectroscopic data.