Variability and the X-ray/UV ratio of active galactic nuclei^⋆

II. Analysis of a low-redshift Swift sample

¹ Dipartimento di Fisica, Università di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
e-mail: fausto.vagnetti@roma2.infn.it
² Dipartimento di Fisica, Università di Roma “La Sapienza”, Piazzale Aldo Moro 2, 00185 Roma, Italy

Received: 25 September 2012
Accepted: 14 December 2012

Abstract

Context. 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).

Aims. 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.

Methods. 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.

Results. 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.