  The UV/optical variations in many AGN are very well correlated, showing
delays which increase with increasing wavelength. It is thought that this is
due to thermal reprocessing of the X-ray emission by the accretion disk. In
this scenario, the variable X-ray flux from the corona illuminates the
accretion disk where it is partially reflected, and partially absorbed and
thermalized in the disk producing a UV/optical reverberation signal. This will
lead to a time lag increasing with wavelength. However, although the shape of
the observed time-lags as a function of wavelength is consistent with the model
predictions, their amplitude suggested a disk which is significantly hotter
than expected. In this work, we estimate the response functions and the
corresponding time lags assuming a standard Novikov-Thorne accretion disk
illuminated by a point-like X-ray source. We take into account all relativistic
effects in the light propagation from the X-ray source to the disk then to the
observer. We also compute the disk reflection, accounting for its ionization
profile. Our results show that thermal reverberation effects are stronger in
sources with large X-ray source height and low accretion rate. We also found
that the time lags increase with height and accretion rate. We apply our model
to NGC 5548 and we show that the observed lags in this source can be explained
by the model, for a source height of $\sim 60 ~\rm r_g$ and an accretion rate
of a few percent of the Eddington limit for a maximally-spinning black hole.
