Binary Black Hole Mergers in Gaseous Environments: "Binary Bondi" and "Binary Bondi-Hoyle-Lyttleton" Accretion

TL;DR

This study uses relativistic hydrodynamic simulations to explore electromagnetic signatures from merging supermassive black hole binaries in gaseous environments, highlighting potential observability of luminosity enhancements.

Contribution

It introduces detailed simulations of black hole mergers in gas clouds considering both stationary and moving binaries, revealing significant electromagnetic luminosity increases.

Findings

Luminosity enhancements up to 3x10^43 erg/s predicted.

Detectable electromagnetic signals possible with LSST at z=1.

Gas accretion rates significantly increase during mergers.

Abstract

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.