  Simultaneous gravitational and electromagnetic wave observations of merging
black hole binaries (BHBHs) can provide unique opportunities to study
gravitation physics, accretion and cosmology. Here we perform fully general
relativistic, hydrodynamic simulations of equal-mass, nonspinning BHBHs
coalescing in a circumbinary disk. We evolve the metric using the
Baumgarte-Shapiro-Shibata-Nakamura (BSSN) formulation of Einstein's field
equations with standard moving puncture gauge conditions. We handle the
hydrodynamics via a high-resolution shock-capturing (HRSC) scheme. We track the
inspiral starting from a binary separation of 10M, where M is the total binary
mass. We take the disks to have an inner radius at R_in~15M to account for the
hollow created by the binary torques. Our disks extend to R=65M and have an
initial scale height of H/R=0.03-0.11. The gas is governed by a Gamma-law EOS,
with Gamma equal to 5/3, 4/3, and 1.1. Disks are allowed to relax in the "early
inspiral" epoch to provide quasistationary realistic initial data. We then
evolve the metric and matter during the "late inspiral and merger" epoch. The
later simulations are designed to track BHBH inspiral following disk-binary
decoupling, through merger and ringdown, terminating before viscosity has time
to fill the hollow about the remnant. We compute the gas flow and accretion
rate and estimate the electromagnetic luminosity due to bremsstrahlung and
synchrotron emission as a perturbation for optically thin disks. The
synchrotron component of the luminosity peaks in the infrared band and should
be detectable by WFIRST and possibly the LSST for a 10^8 M_sun binary embedded
in a disk with a density n~10^12/cm^3 at z=1, beginning with a maximum value of
$L~10^46 n_12^2 M_8^3 erg/s at decoupling, and decreasing steadily over a
timescale of ~100 M_8 hours to a value of L~10^45 n_12^2 M_8^3 erg/s at merger.
