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

Context. The well established negative correlation between the alpha(OX) spectral slope and the optical/ultraviolet (UV) luminosity, a by-product of the relation between X-rays and optical/UV luminosity, is affected by relatively large dispersion. The main contributors to this dispersion can be variability in the X-ray/UV ratio and/or changes in fundamental physical parameters. Aims. We want to quantify the contribution from variability within single sources (intra-source dispersion) and that from variations of other quantities different from source to source (inter-source dispersion). Methods. We use archival data from the XMM-Newton Serendipitous Source Catalog (XMMSSC) and from the XMM-OM Serendipitous Ultraviolet Source Survey (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 that due to absorptions in both X-ray and optical/UV bands. We use the structure function (SF) to estimate the contribution from variability to the dispersion. We analyse the dependence of the residuals of the relation on various physical parameters in order to characterise the inter-source dispersion. Results. We find a total dispersion of sigma similar to 0.12 and find that intrinsic variability contributes 56% of the variance of the alpha(OX) - L-UV relation. If we select only sources with a larger number of observational epochs (>= 3) the dispersion of the relation decreases by approximately 15%. We find weak but significant dependencies of the residuals of the relation on black-hole mass and on Eddington ratio, which are also confirmed by a multivariate regression analysis of alpha(OX) as a function of UV luminosity and black-hole mass and/or Eddington ratio. We find a weak positive correlation of both the alpha(OX) index and the residuals of the alpha(OX) - L-UV relation with inclination indicators, such as the full width at half maximum (H beta) and the equivalent width (EW)[OIII], suggesting a weak increase of X-ray/UV ratio with the viewing angle. This suggests the development of new viewing angle indicators possibly applicable at higher redshifts. Moreover, our results suggest the possibility of selecting a sample of objects, based on their viewing angle and/or black-hole mass and Eddington ratio, for which the alpha(OX) - L-UV relation is as tight as possible, in light of the use of the optical/UV - X-ray luminosity relation to build a distance modulus (DM)-z plane and estimate cosmological parameters.