In this figure we compare the results of the L_X − L_UV relation when using photometric or spectroscopic luminosity measurements for the X-ray luminosity. Left panel: fit of the L_X − L_UV relation using the photometric X-ray luminosities, for the sample of 193 objects described in Section 5. A standard ΛCDM model is used to estimate luminosities from fluxes. We obtain a slope α = 0.58 ± 0.03. Right panel: same as the left panel, but using the spectroscopic estimate of the 2 keV luminosity. The obtained slope is α = 0.59 ± 0.04, fully consistent with the photometric one.