The X-ray/UV ratio in active galactic nuclei: dispersion and variability

The relation between the $alpha_OX$ index and the optical/UV luminosity ($L_UV$), a by product of the X-ray - optical/UV luminosity relation, is affected by a relatively large dispersion, due to variability in the $alpha_OX$ within single sources (intra-source dispersion) and variations of fundamental physical parameters from source to source (inter-source dispersion). We use archival data from the XMMSSC and from the XMMOM-SUSS3. We select a sub-sample in order to decrease the dispersion of the relation due to the presence of Radio-Loud and Broad Absorption Line objects, and to absorptions in both X-ray and optical/UV bands. We analyse the dependence of the residuals of the relation on various physical parameters in order to characterise the inter-source dispersion. We find a total dispersion of ~0.12 and, using the Structure Function, we find that intrinsic variability contributes for 56% of the variance of the relation. We find weak but significant dependences of the residuals of the relation on black-hole (BH) mass and on Eddington ratio, confirmed by a multivariate regression analysis of $alpha_OX$ as a function of optical/UV luminosity and the above quantities. We find a weak positive correlation of both the $alpha_OX$ and the residuals of the relation with inclination indicators (FWHM(H$eta$) and EW[O$_III$]) suggesting a weak increase of X-ray/UV ratio with the viewing angle. Our results suggest the possibility of selecting a sample of objects, based on their viewing angle and/or BH mass and Eddington ratio, for which the $alpha_OX-L_UV$ relation is as tight as possible, in light of the use of the X-ray - optical/UV luminosity relation to build a distance modulus (DM) - $z$ plane and estimate cosmological parameters.