Merging supermassive black hole–black hole 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 black hole–black hole binaries embedded in gas clouds. We evolve the metric using the Baumgarte-Shapiro-Shibata-Nakamura formulation with standard moving puncture gauge conditions and handle the hydrodynamics via a high-resolution shock-capturing 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 the Large Synoptic Survey Telescope for a ${10}^6{M}_{\odot }$ binary in a hot gas cloud of density $n\sim 10{\mathrm{cm}}^{-3}$ and temperature $T\sim {10}^6\mathrm{K}$ at $z=1$, reaching a maximum of $L\sim 3\times {10}^{43}\mathrm{erg}{\mathrm{s}}^{-1}$, with the emission peaking in the visible band, and lasting for $\sim 1$ hour.

• Received 14 December 2009

DOI:https://doi.org/10.1103/PhysRevD.81.084008