  Merging supermassive black hole-black hole (BHBH) binaries produced in galaxy
mergers are promising sources of detectable gravitational waves. If such a
merger takes place in a gaseous environment, there is a possibility of a
simultaneous detection of electromagnetic and gravitational radiation, as the
stirring, shock heating and accretion of the gas may produce variability and
enhancements in the electromagnetic flux. Such a simultaneous detection can
provide a wealth of opportunities to study gravitational physics, accretion
physics, and cosmology. We investigate this scenario by performing fully
general relativistic, hydrodynamic simulations of merging, equal-mass,
nonspinning BHBH binaries embedded in gas clouds. We evolve the metric using
the BSSN formulation with standard moving puncture gauge conditions and handle
the hydrodynamics via a high-resolution shock-capturing (HRSC) scheme. We
consider both "binary Bondi accretion" in which the binary is at rest relative
to the ambient gas cloud, as well as "binary Bondi-Hoyle-Lyttleton accretion"
in which the binary moves relative to the gas cloud. The gas cloud is assumed
to be homogeneous far from the binary and governed by a \Gamma-law equation of
state. We vary \Gamma between 4/3 and 5/3. For each simulation, we compute the
gas flow and accretion rate and estimate the electromagnetic luminosity due to
bremsstrahlung and synchrotron emission. We find evidence for significant
enhancements in both the accretion rate and luminosity over values for a single
black hole of the same mass as the binary. We estimate that this luminosity
enhancement should be detectable by LSST for a 10^6 M_sun binary in a hot gas
cloud of density n~10/cm^3 and temperature T~10^6 K at z=1, reaching a maximum
of L~3x10^43 erg/s, with the emission peaking in the visible band.
