  The observed relation between the X-ray radiation from AGNs, originating in
the corona, and the optical/UV radiation from the disk is usually described by
the anticorrelation between the UV to X-ray slope alpha_ox and the UV
luminosity. Many factors can affect this relation, including: enhanced X-ray
emission associated with the jets of radio-loud AGNs; X-ray absorption
associated with the UV Broad Absorption Line (BAL) outflows; other X-ray
absorption not associated with BALs; intrinsic X-ray weakness; UV and X-ray
variability, and non-simultaneity of UV and X-ray observations. The separation
of these effects provides information about the intrinsic alpha_ox-L_UV
relation and its dispersion, constraining models of disk-corona coupling. We
extract simultaneous data from the second XMM-Newton serendipitous source
catalogue and the XMM-Newton Optical Monitor Serendipitous UV Source Survey
Catalog, and derive the single-epoch alpha_ox indices. We use ensemble
structure functions to analyse multi-epoch data. We confirm the anticorrelation
of alpha_ox with L_UV, and do not find any evidence of a dependence of alpha_ox
on z. The dispersion in our simultaneous data (0.12) is not significantly
smaller than in previous non-simultaneous studies, suggesting that "artificial
alpha_ox variability" introduced by non-simultaneity is not the main cause of
dispersion. "Intrinsic alpha_ox variability", i.e., the true variability of the
X-ray to optical ratio, is instead important, and accounts for ~30% of the
total variance, or more. "Inter-source dispersion", due to intrinsic
differences in the average alpha_ox values from source to source, is also
important. The dispersion introduced by variability is mostly caused by the
long timescale variations, which are expected to be driven by the optical
variations.
