Variability and the X-ray/UV ratio of active galactic nuclei ? II. Analysis of a low-redshift Swift sample

Context. 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). Aims. We aim to evaluate the intrinsic alpha(ox) variations in individual objects and their effect on the dispersion of the alpha(ox) - L-UV anticorrelation. Methods. We used simultaneous UV/X-ray data from Swift observations of a low-redshift sample to derive the epoch-dependent alpha(ox)(t) indices. We corrected for the host galaxy contribution by a spectral fit of the optical/UV data. We computed ensemble structure functions to analyse the variability of multi-epoch data. Results. We find a strong intrinsic alpha(ox) variability, which significantly contributes (similar to 40% 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 active galactic nuclei, and are neither caused by the high fraction of strongly variable narrow line Seyfert 1 galaxies, nor by dilution of the optical variability by the host galaxies. Dilution instead affects the slope of the anticorrelation, which steepens, once corrected, and becomes similar to higher luminosity sources. The structure function of alpha(ox) increases with the time lag up to about one month. This indicates the important contribution of the intermediate-to-long timescale variations, which are possibly generated in the outer parts of the accretion disk.