The Decoupling of Binaries from Their Circumbinary Disks

Abstract

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 ∼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 ∼3, and illustrate the utility of a velocity-based decoupling criterion. High-viscosity (ν ≳ 0.03 GM c−2) circumbinary systems decouple late (a b ≲ 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.

The Evolution of Accreting Binaries: From Brown Dwarfs to Supermassive Black Holes

“…and we set β = 10, r d = 9.5a b , and l d = 0.5a b . These damping terms serve to suppress m = 4 instabilities related to our use of a Cartesian grid and have been employed previously in Dittmann et al (2023). The outer boundary in our simulations is close enough to the binary that in some of our highest mass ratio simulations these damping terms noticeably damp the disk eccentricity.…”

Tracking Supermassive Black Hole Mergers from kpc to sub-pc Scales with AXIS

“…For instance, in the final days before coalescence, the X-ray bright minidisks shrink as the binary separation shrinks and are eventually disrupted, causing a sudden drop in X-ray flux of around a few orbits before a merger, while the optical flux, which is dominated by the outer circumbinary disk, remains steady; the system then gradually re-brightens post-merger [97]. Other simulations also show that the rapidly inspiraling binary can decouple from the circumbinary disk, which may also cause a sharp decline in flux at short wavelengths [146]. The synergy here with an optical survey like LSST is clear: the candidate can be identified by the sudden disappearance of its X-ray flux accompanied by a steady optical emission.…”

Disk-induced Binary Precession: Implications for Dynamics and Multimessenger Observations of Black Hole Binaries

“…Disk-induced precession in the late stages of massive binary evolution could also potentially have important consequences for near-equal-mass binary inspiral events that will be detected by LISA. Binaries in the LISA band (10 3  M/M e  10 7 ) can stay coupled to their circumbinary disks anywhere from ∼1 yr down to minutes prior to merger (Major Krauth et al 2023a;Dittmann et al 2023b); or they may not decouple at all (Avara et al 2023). Therefore, even though the disk-induced precession in band is likely small compared to that from GR, the responsible disk forces could still feasibly alter gravitational wave emission and could leave a detectable imprint on LISA signals.…”