  The UV/optical and X-ray variability of active galactic nuclei (AGN) have
long been expected to be well correlated as a result of the X-ray illumination
of the accretion disk. Recent monitoring campaigns of nearby AGN, however,
found that their X-ray and UV/optical emission are only moderately correlated,
challenging the aforementioned paradigm. In this work, we aim to demonstrate
that due to the definition of the cross correlation function, a low UV/X-ray
correlation is well expected in the case of an X-ray illuminated accretion
disk, when the dynamic variability of the X-ray source is taken into account.
In particular, we examine how the variability of the geometric or physical
configuration of the X-ray source affects the expected correlation. Variations
of the geometric configuration are found to produce a range of UV/X-ray cross
correlations, which match well the observed values, while they result in high
correlation between the UV and optical variability, reconciling the observed
results with theoretical predictions. We conclude that the detection of a low
UV/X-ray correlation does not contradict the assumption of the UV/optical
variability being driven by the X-ray illumination of the disk, and we discuss
the implications of our results for correlation studies.
