X-ray/UV Swift monitoring of NGC 4151

The three hardest X-ray bands (>2.5keV) are all strongly correlated with no measurable interband lag, while the two softer bands show lower variability and weaker correlations. The UV/optical bands are significantly correlated with the X-rays, lagging ~3-4days behind the hard X-rays.

The variability within the UV/optical bands is also strongly correlated, with the UV appearing to lead the optical by ~0.5-1days. This combination of >~3day lags between the X-rays and UV and <~1day lags within the UV/optical appears to rule out the "lamp-post" reprocessing model in which a hot, X-ray emitting corona directly illuminates the accretion disk, which then reprocesses the energy in the UV/optical. Instead, these results appear consistent with the Gardner & Done (2017MNRAS.470.3591G) picture in which two separate reprocessings occur: first, emission from the corona illuminates an extreme-UV-emitting toroidal component that shields the disk from the corona; this then heats the extreme-UV component, which illuminates the disk and drives its variability.