A synchrotron self-Compton -- disk reprocessing model for optical/X-ray
  correlation in black hole X-ray binaries
  Physical picture of the emission mechanisms operating in the X-ray binaries
was put under question by the simultaneous optical/X-ray observations with high
time resolution. The light curves of the two energy bands appeared to be
connected and the cross-correlation functions observed in three black hole
binaries exhibited a complicated shape. They show a dip of the optical emission
a few seconds before the X-ray peak and the optical flare just after the X-ray
peak. This behavior could not be explained in terms of standard optical
emission candidates (e.g., emission from the cold accretion disk or a jet). We
propose a novel model, which explains the broadband optical to the X-ray
spectra and the variability properties. We suggest that the optical emission
consists of two components: synchrotron radiation from the non-thermal
electrons in the hot accretion flow and the emission produced by reprocessing
of the X-rays in the outer part of the accretion disk. The first component is
anti-correlated with the X-rays, while the second one is correlated, but
delayed and smeared relative to the X-rays. The interplay of the components
explains the complex shape of the cross-correlation function, the features in
the optical power spectral density as well as the time lags.
