Ultraviolet and X-ray variability of active galactic nuclei with Swift

Abstract

We analyse a sample of 21 active galactic nuclei (AGN) using data from the Swift satellite to study the variability properties of the population in the X-ray, UV and optical band. We find that the variable part of the UV-optical emission has a spectrum consistent with a powerlaw, with an average index of $-2.21\pm 0.13$, as would be expected from central illumination of a thin disc (index of -7/3). We also calculate the slope of a powerlaw from UV to X-ray variable emission, ${\alpha }_{\mathrm{OX},\mathrm{Var}}$; the average for this sample is ${\alpha }_{\mathrm{OX},\mathrm{Var}}=-1.06\pm 0.04$. The anticorrelation of ${\alpha }_{\mathrm{OX}}$ with the UV luminosity, $L_{\mathrm{UV}}$, previously found in the average emission is also present in the variable part: ${\alpha }_{\mathrm{OX},\mathrm{Var}}=(-0.177\pm 0.083)\log (L_{\nu ,\mathrm{Var}}(2500\mathring{A}))+(3.88\pm 2.33)$. Correlated variability between the emission in X-rays and UV is detected significantly for 9 of the 21 sources. All these cases are consistent with the UV lagging the X-rays, as would be seen if the correlated UV variations were produced by the reprocessing of X-ray emission. The observed UV lags are tentatively longer than expected for a standard thin disc.

14 pages, 8 figures, 4 tables in main text, plus 15 pages of figures in appendices. Accepted for publication in MNRAS

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