Hot accretion flow in black hole binaries: a link connecting X-rays to
  the infrared
  Multiwavelength observations of Galactic black hole transients have opened a
new path to understanding the physics of the innermost parts of the accretion
flows. While the processes giving rise to their X-ray continuum have been
studied extensively, the emission in the optical and infrared (OIR) energy
bands was less investigated and remains poorly understood. The standard
accretion disc, which may contribute to the flux at these wavelengths, is not
capable of explaining a number of observables: the infrared excesses, fast OIR
variability and a complicated correlation with the X-rays. It was suggested
that these energy bands are dominated by the jet emission, however, this
scenario does not work in a number of cases. We propose here an alternative,
namely that most of the OIR emission is produced by the extended hot accretion
flow. In this scenario, the OIR bands are dominated by the synchrotron
radiation from the non-thermal electrons. An additional contribution is
expected from the outer irradiated part of the accretion disc heated by the
X-rays. We discuss properties of the model and compare them to the data. We
show that the hot-flow scenario is consistent with many of the observed
spectral data, at the same time naturally explaining X-ray timing properties,
fast OIR variability and its correlation with the X-rays.
