2013A&A...550A..71V

Variability, both in X-ray and optical/UV, affects the well-known anti-correlation between the α_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 α_ox values from source to source (inter-source dispersion). We aim to evaluate the intrinsic α_ox variations in individual objects and their effect on the dispersion of the α_ox-L_UV anti-correlation. We used simultaneous UV/X-ray data from Swift observations of a low-redshift sample to derive the epoch-dependent α_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. We find a strong intrinsic α_ox variability, which significantly contributes (∼40% of the total variance) to the dispersion of the α_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 anti-correlation, which steepens, once corrected, and becomes similar to higher luminosity sources. The structure function of α_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.