The merger of spinning, accreting supermassive black hole binaries (2509.10319v1)

Abstract: Because they are likely to accrete substantial amounts of interstellar gas, merging supermassive binary black holes are expected to be strong multimessenger sources, radiating gravitational waves, photons from thermal gas, and photons from relativistic electrons energized by relativistic jets. Here we report on a numerical simulation that covers the late inspiral, merger, and initial postmerger phase of such a system where both black holes have the same mass and spin, and both spin axes are parallel to the orbital angular momentum. The simulation incorporates both 3D general relativistic magnetohydrodynamics and numerical relativity. The thermal photon power during the late inspiral, merger, and immediate postmerger phases is drawn from strong shocks rather than dissipation of turbulence inside a smoothly structured accretion disk as typically found around accreting single black holes. We find that the thermal photon and jet Poynting flux outputs are closely related in time, and we posit a mechanism that enforces this relation. The power radiated in both photons and jets diminishes gradually as merger is approached, but jumps sharply at merger to a noisy plateau. Such a distinct lightcurve should aid efforts to identify supermassive black hole mergers, with or without accompanying gravitational wave detections.

• The paper demonstrates that high-spin configurations result in minidisks with 4–5 times higher mass and lengthened inspiral phases due to spin–orbit coupling.
• It applies fully coupled 3D GRMHD and numerical relativity to capture detailed gas dynamics, shock formation, and jet launching processes during merger.
• The study finds a stable linear correlation between photon and jet Poynting luminosities, providing actionable EM markers for SMBBH merger detection.

Numerical Study of Spinning, Accreting Supermassive Black Hole Binary Mergers

Introduction

This paper presents a comprehensive numerical investigation of the late inspiral, merger, and immediate postmerger phases of equal-mass, spinning supermassive black hole binaries (SMBBHs) embedded in accreting circumbinary disks. The paper employs fully coupled 3D general relativistic magnetohydrodynamics (GRMHD) and numerical relativity, focusing on the case where both black holes possess high, aligned spins. The work addresses the electromagnetic (EM) and jet signatures associated with these mergers, with particular attention to the interplay between photon and Poynting luminosities, the evolution of minidisks, and the role of shocks in postmerger emission.

Simulation Setup and Methodology

The simulations utilize the IllinoisGRMHD code, integrating the Einstein field equations with GRMHD to capture the dynamics of both spacetime and magnetized plasma. The initial conditions are based on a relaxed circumbinary disk (CBD) state, with both black holes assigned a dimensionless spin parameter of 0.8, aligned with the orbital angular momentum. The computational grid is refined to resolve the smaller horizons of spinning black holes, ensuring accurate treatment of near-horizon physics and jet launching regions.

The simulation tracks the evolution of mass density, magnetic fields, photon emissivity, and jet structure from early inspiral through merger and into the postmerger regime.

Figure 1: Mass density, Poynting vector magnitude with magnetic field lines, and photon emissivity at three key epochs: early inspiral, pre-merger, and post-merger.

Evolution of Minidisks and Gas Dynamics

During the early inspiral, prominent minidisks form around each black hole, exhibiting alternating "disk-like" and "stream-like" states. The presence of spin increases the initial minidisk mass by a factor of 4–5 compared to nonspinning cases, attributed to the reduced ISCO radius. The "hangup" effect, resulting from spin-orbit coupling, prolongs the inspiral and sustains the minidisks for several thousand $M$ longer than in nonspinning binaries.

Mass exchange ("sloshing") between minidisks is observed, but as the binary approaches merger, the minidisks dissolve. Post-dissolution, gas heating becomes highly localized in narrow shock fronts, indicating a transition from turbulent dissipation to shock-dominated heating.

Jet Launching and Magnetic Field Evolution

Accretion-driven magnetic flux accumulation on the black hole horizons enables the formation of relativistic, Poynting-flux dominated jets along the spin axes. During inspiral, the jets remain distinct, separated by a low-magnetization layer, and merge at larger distances from the orbital plane. The maximum Poynting flux in the jets is consistent with previous results for lower-spin binaries.

The magnetic field topology is influenced by the aligned spins and accreted field polarity, leading to regions of magnetic reconnection near the orbital axis.

Electromagnetic and Jet Luminosity Evolution

The photon luminosity and jet Poynting flux both exhibit gradual declines during inspiral, punctuated by temporary recoveries. Notably, the decline in photon luminosity from the innermost region ($r \leq 15\,M$) is less pronounced in the spinning case (factor $\sim$4) compared to the nonspinning case (factor $\sim$8). At merger, both luminosities experience a sharp increase, returning to or exceeding their initial levels.

Figure 2: Evolution of mass within $r \leq 15\,M$, accretion rate, photon luminosity (inner and outer regions), and jet Poynting flux.

A key result is the strong linear correlation between photon and Poynting luminosities during inspiral, with their ratio remaining nearly constant ($$L_\text{photons}/L_\text{Poynting} \approx 0.26 \pm 0.04$$) over several thousand $M$ prior to merger. This correlation is absent in nonspinning binaries, where jet power is negligible.

Figure 3: Ratio of photon luminosity (from $r \leq 15\,M$ and $r \leq 100\,M$) to jet Poynting flux at $r=100\,M$ as a function of time.

Physical Interpretation and Theoretical Implications

The observed coupling between photon and Poynting luminosities is interpreted as a consequence of thermal balance in the inner cavity, where the cooling time is independent of the heating rate and the magnetic flux is confined by thermal rather than ram pressure. The scaling $L_\text{photons} \propto M_\text{gas}^{0.32}$ suggests that the heating rate per unit mass decreases with increasing gas mass, possibly due to shock suppression in more massive minidisks.

After merger, both luminosities become uncorrelated with accretion rate and gas mass, and the radiative efficiency ($L_\text{photons}/\dot{M} \sim 1$–2) exceeds standard expectations for relativistic accretion, indicating dominance of relativistic shocks in a highly irregular density field.

The persistence of significant EM and jet power throughout the merger, and the immediate postmerger rise, contradicts predictions from Newtonian 2D hydrodynamics that suggest prolonged low-luminosity phases. The results demonstrate that shocks and magnetic processes can sustain observable emission even after the disappearance of minidisks.

Observational and Astrophysical Implications

The distinct lightcurve signature—gradual decline followed by a sharp rise in both photon and jet luminosities—provides a potential EM marker for SMBBH mergers, independent of GW detection. The timescales for these features (months for decline, days for rise for $M \sim 10^8 M_\odot$) are accessible to current and future time-domain surveys.

The close coupling of photon and jet emission implies that multiwavelength monitoring (thermal, synchrotron, inverse Compton) could enhance the identification and characterization of SMBBH mergers. The abrupt change in emission region at merger also suggests a corresponding spectral transition, offering an additional diagnostic.

These findings have direct relevance for the planning and interpretation of multimessenger campaigns, particularly in the context of future spaceborne GW observatories such as LISA, where EM counterparts will be critical for source localization and population studies.

Conclusion

This paper provides a detailed numerical analysis of spinning, accreting SMBBH mergers, revealing robust EM and jet signatures that persist through merger and are tightly coupled during inspiral. The results challenge previous expectations of EM quiescence and highlight the importance of relativistic shocks and magnetic confinement in shaping the observable properties of these systems. The predicted lightcurve and spectral features offer promising avenues for the identification of SMBBH mergers and the advancement of multimessenger astrophysics. Future work should extend these simulations to a broader range of spin configurations, mass ratios, and disk properties to further elucidate the diversity of EM signatures from SMBBH mergers.