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Abstract We demonstrate that gas disks around binary systems might deliver gas to the binary components only when the circumbinary disk is relatively warm. We present new grid-based hydrodynamics simulations, performed with the binary on the grid and a locally isothermal equation of state, in which the binary is seen to functionally “stop accreting” if the orbital Mach number in the disk exceeds a threshold value of about 40. Above this threshold, the disk continues to extract angular momentum from the binary orbit, but it delivers very little mass to the black holes and instead piles up mass in a ring surrounding the binary. This ring will eventually become viscously relaxed and deliver mass to the binary at the large-scale inflow rate. However, we show that the timescale for such relaxation can far exceed the implied binary lifetime. We demonstrate that the ability of a binary–disk system to equilibrate is dependent on the efficiency at which accretion streams deposit mass onto the binary, which, in turn is highly sensitive to the thermodynamic conditions of the inner disk. If disks around massive black hole binaries do operate in such nonaccreting regimes, it suggests these systems may be dimmer than their single black hole counterparts but could exhibit dramatic rebrightening after the black holes inspiral and merge. This dimming begins in the UV/optical and could completely choke high-energy emission, such that these systems would likely be intrinsically X-ray weak with reddened continua, potentially resembling the spectra of “little red dots” recently identified in JWST observations. 

Abstract We analyze accretion-rate time series for equal-mass binaries in coplanar gaseous disks spanning a continuous range of orbital eccentricities up to 0.8 for both prograde and retrograde systems. The dominant variability timescales match those of previous investigations; the binary orbital period is dominant for prograde binaries withe≳ 0.1, with a 5 × longer “lump” period taking over fore≲ 0.1. This lump period fades and drops from 5 × to 4.5 × the binary period aseapproaches 0.1, where it vanishes. For retrograde orbits, the binary orbital period dominates ate≲ 0.55 and is accompanied by a 2 × longer timescale periodicity at higher eccentricities. The shape of the accretion-rate time series varies with binary eccentricity. For prograde systems, the orientation of an eccentric disk causes periodic trading of accretion between the binary components in a ratio that we report as a function of binary eccentricity. We present a publicly available tool,binlite, that can rapidly (≲0.01 s) generate templates for the accretion-rate time series onto either binary component for choice of binary eccentricity below 0.8. As an example use case, we build lightcurve models where the accretion rate through the circumbinary disk and onto each binary component sets contributions to the emitted specific flux. We combine these rest-frame, accretion-variability lightcurves with observer-dependent Doppler boosting and binary self-lensing. This allows a flexible approach to generating lightcurves over a wide range of binary and observer parameter space. We envisionbinliteas the access point to a living database that will be updated with state-of-the-art hydrodynamical calculations as they advance. 

Abstract Many studies have recently documented the orbital response of eccentric binaries accreting from thin circumbinary disks, characterizing the change in the binary semimajor axis and eccentricity. We extend these calculations to include the precession of the binary’s longitude of periapse induced by the circumbinary disk, and we characterize this precession continuously with binary eccentricitye_bfor equal mass components. This disk-induced apsidal precession is prograde with a weak dependence on the binary eccentricity whene_b≲ 0.4 and decreases approximately linearly fore_b≳ 0.4; yet at alle_bbinary precession is faster than the rates of change to the semimajor axis and eccentricity by an order of magnitude. We estimate that such precession effects are likely most important for subparsec separated binaries with masses ≲10⁷M_⊙, like LISA precursors. We find that accreting, equal-mass LISA binaries withM< 10⁶M_⊙(and the most massiveM∼ 10⁷M_⊙binaries out toz∼ 3) may acquire a detectable phase offset due to the disk-induced precession. Moreover, disk-induced precession can compete with general relativistic precession in a vacuum, making it important for observer-dependent electromagnetic searches for accreting massive binaries—like Doppler boost and binary self-lensing models—after potentially only a few orbital periods. 

Abstract We have performed numerical calculations of a binary interacting with a gas disk, using 11 different numerical methods and a standard binary−disk setup. The goal of this study is to determine whether all codes agree on a numerically converged solution and to determine the necessary resolution for convergence and the number of binary orbits that must be computed to reach an agreed-upon relaxed state of the binary−disk system. We find that all codes can agree on a converged solution (depending on the diagnostic being measured). The zone spacing required for most codes to reach a converged measurement of the torques applied to the binary by the disk is roughly 1% of the binary separation in the vicinity of the binary components. For our disk model to reach a relaxed state, codes must be run for at least 200 binary orbits, corresponding to about a viscous time for our parameters, 0.2(a²Ω_B/ν) binary orbits, whereνis the kinematic viscosity. The largest discrepancies between codes resulted from the dimensionality of the setup (3D vs. 2D disks). We find good agreement in the total torque on the binary between codes, although the partition of this torque between the gravitational torque, orbital accretion torque, and spin accretion torque depends sensitively on the sink prescriptions employed. In agreement with previous studies, we find a modest difference in torques and accretion variability between 2D and 3D disk models. We find cavity precession rates to be appreciably faster in 3D than in 2D. 

Abstract Linear analysis of gas flows around orbiting binaries suggests that a centrifugal barrier ought to clear a low-density cavity around the binary and inhibit mass transfer onto it. Modern hydrodynamics simulations have confirmed the low-density cavity, but show that any mass flowing from large scales into the circumbinary disk is eventually transferred onto the binary components. Even though many numerical studies confirm this picture, it is still not understood precisely how gas parcels overcome the centrifugal barrier and ultimately accrete. We present a detailed analysis of the binary accretion process, using an accurate prescription for evolving grid-based hydrodynamics with Lagrangian tracer particles that track the trajectories of individual gas parcels. We find that binary accretion can be described in four phases: (1) gas is viscously transported through the circumbinary disk up to the centrifugal barrier at the cavity wall, (2) the cavity wall is tidally distorted into accretion streams consisting of near-ballistic gas parcels on eccentric orbits, (3) the portion of each stream moving inwards of an accretion horizon radius r ¯ ≃ a —the radius beyond which no material is returned to the cavity wall—becomes bound to a minidisk orbiting an individual binary component, and (4) the minidisk gas accretes onto the binary component through the combined effect of viscous and tidal stresses.