  The new multi-wavelength monitoring campaign on NGC 5548 shows clearly that
the variability of the UV/optical lightcurves lags by progressively longer
times at longer wavelengths, as expected from reprocessing of an optically
thick disk, but that the timescales are longer than expected for a standard
Shakura-Sunyaev accretion disc. We build a full spectral-timing reprocessing
model to simulate the UV/optical lightcurves of NGC 5548. We show that disc
reprocessing of the observed hard X-ray lightcurve produces optical lightcurves
with too much fast variability as well as too short a lag time. Supressing the
fast variability requires an intervening structure preventing the hard X-rays
from illuminating the disc. We propose this is the disc itself, perhaps due to
atomic processes in the UV lifting the photosphere, increasing the
scale-height, making it less dense and less able to thermalise, so that it
radiates low temperature Comptonised emission as required to produce the soft
X-ray excess. The outer edge of the puffed-up Comptonised disc region emits FUV
flux, and can illuminate the outer thin blackbody disc but while this gives
reprocessed variable emission which is much closer to the observed UV and
optical lightcurves, the light travel lags are still too short to match the
data. We reverse engineer a solution to match the observations and find that
the luminosity and temperature of the lagged emission is not consistent with
material at the light travel lag distance responding to the irradiating flux
(either FUV or X-ray) from the AGN. We conclude that the UV/optical lags of NGC
5548 are not the light travel time from X-ray reprocessing, nor the light
travel time from FUV reprocessing, but instead could be the timescale for the
outer blackbody disc vertical structure to respond to the changing FUV
illumination.
