  We model the electromagnetic signatures of massive black hole binaries
(MBHBs) with an associated gas component. The method comprises numerical
simulations of relativistic binaries and gas coupled with calculations of the
physical properties of the emitting gas. We calculate the UV/X-ray and the
Halpha light curves and the Halpha emission profiles. The simulations are
carried out with a modified version of the parallel tree SPH code Gadget. The
heating, cooling, and radiative processes are calculated for two different
physical scenarios, where the gas is approximated as a black-body or a solar
metallicity gas. The calculation for the solar metallicity scenario is carried
out with the photoionization code Cloudy. We focus on sub-parsec binaries which
have not yet entered the gravitational radiation phase. The results from the
first set of calculations, carried out for a coplanar binary and gas disk,
suggest that there are pronounced outbursts in the X-ray light curve during
pericentric passages. If such outbursts persist for a large fraction of the
lifetime of the system, they can serve as an indicator of this type of binary.
The predicted Halpha emission line profiles may be used as a criterion for
selection of MBHB candidates from existing archival data. The orbital period
and mass ratio of a binary may be inferred after carefully monitoring the
evolution of the Halpha profiles of the candidates. The discovery of sub-parsec
binaries is an important step in understanding of the merger rates of MBHBs and
their evolution towards the detectable gravitational wave window.
