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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 present a theoretical study of the gravitational wave (GW) driven inspirals of accreting black hole binaries with mass $$M = 10^7 M_\odot$$ and mass ratios between $$10^{-3}$$ and $$10^{-1}$$. Our results are based on analytic estimates, and grid-based hydrodynamics simulations run for many thousands of binary orbits before the merger. We show that the GW inspiral is evident in the light curves and colour evolution of a binary-hosting quasar over years to decades before a merger. The long-term electromagnetic (EM) signature is characterized by a gradual UV brightening and X-ray dimming, followed by an X-ray disappearance hours to days before the GW burst, and finally, a years-like re-brightening as the disc relaxes and refuels the remnant black hole. These time-scales are surprisingly insensitive to the normalization of the kinematic viscosity in the disc. The spectrum of quasi-thermal disc emission shows two peaks: one in the UV and another in the X-ray, associated with the outer and circum-secondary discs, respectively; emission from the inner disc is suppressed because the secondary consumes most of the inflowing gas. We discuss implications for real-time and archival EM follow-up of GW bursts detected by LISA. 

ABSTRACT Using grid-based hydrodynamics simulations and analytic modeling, we compute the electromagnetic (EM) signatures of gravitational wave (GW) driven inspirals of massive black hole binaries that accrete gas from circumbinary discs, exploring the effects of varying gas temperatures, viscosity laws, and binary mass ratios. Our main finding is that active galactic nuclei (AGNs) that host inspiraling binaries can exhibit two sub-types of long-term secular variability patterns: Type-A events which dim before merger and brighten afterward, and Type-B events which brighten before merger and dim afterward. In both types, the merger coincides with a long-lasting chromatic change of the AGN appearance. The sub-types correspond to the direction of angular momentum transfer between the binary and the disc, and could thus have correlated GW signatures if the gas-induced torque can be inferred from GW phase drift measurements by LISA. The long-term brightness trends are caused by steady weakening of the disc-binary torque that accompanies orbital decay, it induces a hysteresis effect whereby the disc ‘remembers’ the history of the binary’s contraction. We illustrate the effect using a reduced model problem of an axisymmetric thin disc subjected at its inner edge to the weakening torque of an inspiraling binary. The model problem yields a new class of self-similar disc solutions, which capture salient features of the multidimensional hydrodynamics simulations. We use these solutions to derive variable AGN disc emission signatures within years to decades of massive black hole binary mergers in AGNs. Spectral changes of Mrk 1018 might have been triggered by an inspiral-merger event.