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 _ { OX,Var } ; the average for this sample is \alpha _ { OX,Var } = -1.06 \pm 0.04 .
The anticorrelation of \alpha _ { OX } with the UV luminosity , L _ { UV } , previously found in the average emission is also present in the variable part : \alpha _ { OX,Var } = ( -0.177 \pm 0.083 ) \log ( L _ { \nu, Var } ( 2500 \text { \AA } ) ) + ( % 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 .