The Decoupling of Binaries from Their Circumbinary Disks

We have investigated, both analytically and numerically, accreting supermassive black hole binaries as they inspiral due to gravitational radiation to elucidate the decoupling of binaries from their disks and inform future multimessenger observations of these systems. Our numerical studies evolve equal-mass binaries from initial separations of 100 GM c(-2) until merger, resolving scales as small as similar to 0.04 GM c(-2), where M is the total binary mass. Our simulations accurately capture the point at which the orbital evolution of each binary decouples from that of its circumbinary disk, and precisely resolve the flow of gas throughout the inspiral. We demonstrate analytically and numerically that timescale-based predictions overestimate the binary separations at which decoupling occurs by factors of similar to 3, and illustrate the utility of a velocity-based decoupling criterion. High-viscosity (nu greater than or similar to 0.03 GM c(-2)) circumbinary systems decouple late (a ( b ) less than or similar to 15 GM c(-2)) and have qualitatively similar morphologies near merger to circumbinary systems with constant binary separations. Lower-viscosity circumbinary disks decouple earlier and exhibit qualitatively different accretion flows, which lead to precipitously decreasing accretion onto the binary. If detected, such a decrease may unambiguously identify the host galaxy of an ongoing event within a LISA error volume. We illustrate how accretion amplitude and variability evolve as binaries gradually decouple from their circumbinary disks, and where decoupling occurs over the course of binary inspirals in the LISA band. We show that, even when dynamically negligible, gas may leave a detectable imprint on the phase of gravitational waves.

Automated summary

We have investigated accreting supermassive black hole binaries as they inspiral due to gravitational radiation to understand decoupling of binaries from disks and inform future observations. Our numerical studies accurately capture the decoupling point and gas flow during the inspiral. We demonstrate that timescale-based predictions overestimate decoupling and illustrate a more accurate velocity-based criterion.