UV/optical disk thermal reverberation in AGN: an in-depth study with an
  analytic prescription for the time-lag spectra
  Several active galactic nuclei show correlated variations in the
ultraviolet/optical range, with time delays increasing at longer wavelengths.
Thermal reprocessing of the X-rays illuminating the accretion disk has been
proposed as a viable explanation. In this scenario, the variable X-ray flux
irradiating the accretion disk is partially reflected in X-rays, and partially
absorbed, thermalized and re-emitted with some delay by the accretion disk at
longer wavelengths. We investigate this scenario assuming an X-ray point-like
source illuminating a standard Novikov-Thorne accretion disk, around a rotating
black hole. We consider all special and general relativistic effects to
determine the incident X-ray flux on the disk and in propagating light from the
source to the disk and to the observer. We also compute the disk reflection
flux taking into consideration the disk ionization. We investigate the
dependence of the disk response function and time lags on various physical
parameters, such as black hole mass and spin, X-ray corona height, luminosity,
and photon index, accretion rate, inclination, and inner/outer disk radii. We
found it is important to consider relativistic effects and the disk ionization
in estimating the disk response. We also found a strong non-linearity between
the X-ray luminosity and the disk response. We present an analytic function for
the time-lags dependence on wavelength, which can be used to fit observed
time-lag spectra. We also estimate the fraction of the reverberation signal
with respect to the total flux and we suggest possible explanation for the lack
of X-ray-ultraviolet/optical correlated variations in a few sources.
