Variability and the X-ray/UV ratio of active galactic nuclei. II.
  Analysis of a low-redshift Swift sample
  Variability, both in X-ray and optical/UV, affects the well-known
anti-correlation between the $\alpha_{ox}$ spectral index and the UV luminosity
of active galactic nuclei, contributing part of the dispersion around the
average correlation ("intra-source dispersion"), in addition to the differences
among the time-average $\alpha_{ox}$ values from source to source
("inter-source dispersion"). We want to evaluate the intrinsic $\alpha_{ox}$
variations in individual objects, and their effect on the dispersion of the
$\alpha_{ox}-L_{UV}$ anti-correlation. We use simultaneous UV/X-ray data from
Swift observations of a low-redshift sample, to derive the epoch-dependent
$\alpha_{ox}(t)$ indices. We correct for the host galaxy contribution by a
spectral fit of the optical/UV data. We compute ensemble structure functions to
analyse variability of multi-epoch data. We find a strong "intrinsic
$\alpha_{ox}$ variability", which makes an important contribution ($\sim40%$ of
the total variance) to the dispersion of the $\alpha_{ox}-L_{UV}$
anti-correlation ("intra-source dispersion"). The strong X-ray variability and
weaker UV variability of this sample are comparable to other samples of low-z
AGNs, and are neither due to the high fraction of strongly variable NLS1s, nor
to dilution of the optical variability by the host galaxies. Dilution affects
instead the slope of the anti-correlation, which steepens, once corrected,
becoming similar to higher luminosity sources. The structure function of
$\alpha_{ox}$ increases with the time lag up to $\sim$1 month. This indicates
the important contribution of the intermediate-long timescale variations,
possibly generated in the outer parts of the accretion disk.
