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ABSTRACT The multimessenger combination of gravitational waves (GWs) from merging massive black hole binaries (MBHBs) and the electromagnetic (EM) counterpart from the surrounding circumbinary disc (CBD) will open avenues to new scientific pursuits. In order to realize this science, we need to correctly localize the host galaxy of the merging MBHB. Multiwavelength, time-dependent EM signatures can greatly facilitate the identification of the unique EM counterpart among many sources in LISA’s localization volume. To this end, we studied merging unequal-mass MBHBs embedded in a CBD using high-resolution 2D simulations, with a $$\Gamma$$-law equation of state, incorporating viscous heating, shock heating, and radiative cooling. We simulate each binary starting from before it decouples from the CBD until just after the merger. We compute EM signatures and identify distinct features before, during, and after the merger. We corroborate previous findings of a several orders of magnitude drop in the thermal X-ray luminosity near the time of merger, but with delayed timing compared to an equal-mass system. The source remains X-ray dark for hours post-merger. Our main results are a potential new signature of a sharp spike in the thermal X-ray emission just before the tell-tale steep drop occurs. This feature may further help to identify EM counterparts of LISA’s unequal MBHBs before merger without the need for extensive pre-merger monitoring. Additionally, we find a role-reversal in which the primary out-accretes the secondary during late inspiral, which may diminish signatures originating from Doppler modulation. 

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. 

Abstract Observations and theory suggest that core-collapse supernovae can span a range of explosion energies, and when sub-energetic the shockwave initiating the explosion can decelerate to speeds comparable to the escape speed of the progenitor. In these cases, gravity will complicate the explosion hydrodynamics and conceivably cause the shock to stall at large radii within the progenitor star. To understand these unique properties of weak explosions, we develop a perturbative approach for modeling the propagation of an initially strong shock into a time-steady, infalling medium in the gravitational field of a compact object. This method writes the shock position and the post-shock velocity, density, and pressure as series solutions in the (time-dependent) ratio of the freefall speed to the shock speed, and predicts that the shock stalls within the progenitor if the explosion energy is below a critical value. We show that our model agrees very well with hydrodynamic simulations, and accurately predicts (for example) the time-dependent shock position and velocity and the radius at which the shock stalls. Our results have implications for black hole formation and the newly detected class of fast X-ray transients (FXTs). In particular, we propose that a “phantom shock breakout”—where the outer edge of the star falls through a stalled shock—can yield a burst of X-rays without a subsequent optical/UV signature, similar to FXTs. This model predicts the rise time of the X-ray burst,t_d, and the mean photon energy,kT, are anticorrelated, approximately as $T\propto t_{\mathrm{d}}^{-5/8}$. 

ABSTRACT The upcoming Laser Interferometer Space Antenna (LISA) is expected to detect gravitational waves (GWs) from massive black hole binaries (MBHB). Finding the electromagnetic (EM) counterparts for these GW events will be crucial for understanding how and where MBHBs merge, measuring their redshifts, constraining the Hubble constant and the graviton mass, and for other novel science applications. However, due to poor GW sky localization, multiwavelength, time-dependent EM models are needed to identify the right host galaxy. We studied merging MBHBs embedded in a circumbinary disc (CBD) using high-resolution two-dimensional simulations, with a Γ-law equation of state, incorporating viscous heating, shock heating, and radiative cooling. We simulate the binary from large separation until after merger, allowing us to model the decoupling of the binary from the CBD. We compute the EM signatures and identify distinct features before, during, and after the merger. Our main result is a multiband EM signature: we find that the MBHB produces strong thermal X-ray emission until 1–2 d prior to the merger. However, as the binary decouples from the CBD, the X-ray-bright minidiscs rapidly shrink in size, become disrupted, and the accretion rate drops precipitously. As a result, the thermal X-ray luminosity drops by orders of magnitude, and the source remains X-ray dark for several days, regardless of any post-merger effects such as GW recoil or mass-loss. Looking for the abrupt spectral change where the thermal X-ray disappears is a tell-tale EM signature of LISA mergers that does not require extensive pre-merger monitoring. 

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. 

Abstract Core-collapse supernovae can display evidence of interaction with preexisting, circumstellar shells of material by rebrightening and forming spectral lines, and can even change types as hydrogen appears in previously hydrogen-poor spectra. However, a recently observed core-collapse supernova—SN 2019tsf—was found to brighten after roughly 100 days after it was first observed, suggesting that the supernova ejecta was interacting with surrounding material, but it lacked any observable emission lines and thereby challenged the standard supernova-interaction picture. We show through linear perturbation theory that delayed rebrightenings without the formation of spectral lines are generated as a consequence of the finite sound-crossing time of the postshock gas left in the wake of a supernova explosion. In particular, we demonstrate that sound waves—generated in the postshock flow as a consequence of the interaction between a shock and a density enhancement—traverse the shocked ejecta and impinge upon the shock from behind in a finite time, generating sudden changes in the shock properties in the absence of ambient density enhancements. We also show that a blast wave dominated by gas pressure and propagating in a wind-fed medium is unstable from the standpoint that small perturbations lead to the formation of reverse shocks within the postshock flow, implying that the gas within the inner regions of these blast waves should be highly turbulent.