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We report on the largest Swift AGN monitoring program, concerning UV/opticalvariability in Seyferts. From 554 observations, over a 750d period, of theSeyfert galaxy NGC 5548, we see (McHardy et al. 2014) a good overallcorrelation between the X-ray and UV/optical bands,particularly on shorttimescales (tens of days). The UVOT bands are found to lag behind X-rays with alag scaling as wavelength to the power 1.23 +/- 0.31, in excellent agreementwith that expected (1.33) if UV/optical variability arises from reprocessing ofX-rays by the accretion disc. However, the observed lags are ~3 times longerthan expected from a standard Shakura-Sunyaev disc, raising real concerns aboutthe detailed validity of this model. The results can be explained with aslightly larger mass and accretion rate, and a hotter disc, or if the disc isclumpy, thereby enhancing the emission from the outer regions.

The largest Swift AGN monitoring campaign: UV/optical variability in NGC 5548