Relativistic Gas Accretion onto Supermassive Black Hole Binaries from Inspiral through Merger (2502.06389v3)

Abstract: Accreting supermassive black hole binaries are powerful multimessenger sources emitting both gravitational and EM radiation. Understanding the accretion dynamics of these systems and predicting their distinctive EM signals is crucial to informing and guiding upcoming efforts aimed at detecting gravitational waves produced by these binaries. To this end, accurate numerical modeling is required to describe both the spacetime and the magnetized gas around the black holes. In this paper, we present two key advances in this field of research. First, we have developed a novel 3D GRMHD framework that combines multiple numerical codes to simulate the inspiral and merger of supermassive black hole binaries starting from realistic initial data and running all the way through merger. Throughout the evolution, we adopt a simple but functional prescription to account for gas cooling through photon emission. Next, we have applied our new computational method to follow the time evolution of a circular, equal-mass, non-spinning black hole binary for ~200 orbits, starting from a separation of 20r_g and reaching the post-merger evolutionary stage of the system. We have shown how mass continues to flow toward the binary even after the binary "decouples" from its surrounding disk, but the accretion rate onto the black holes diminishes. We have identified how the minidisks orbiting each black hole are slowly drained and eventually dissolve as the binary compresses. We confirm previous findings that the system's luminosity decreases by only a factor of a few during inspiral; however, we observe an abrupt increase by ~50% in this quantity at the time of merger, likely accompanied by an equally abrupt change in spectrum. Finally, we have demonstrated that during the inspiral, fluid ram pressure regulates the fraction of the magnetic flux transported to the binary that attaches to the black holes' horizons.

• The paper presents a novel 3D GRMHD simulation tracking gas dynamics in equal-mass SMBH binaries over approximately 200 orbits.
• It demonstrates that tidal forces disrupt coherent minidisks into dispersed flows with a slight decline in accretion during decoupling.
• Results reveal a ~50% boost in EM luminosity and enhanced Poynting flux at merger, implicating relativistic jet formation detectable by multi-messenger observations.

Overview of Relativistic Gas Accretion onto Supermassive Black Hole Binaries

The paper "Relativistic Gas Accretion onto Supermassive Black Hole Binaries from Inspiral through Merger" presents an extensive investigation into the dynamics underpinning the accretion processes around supermassive black hole binaries throughout their inspiral and eventual merger stages. This paper is vital due to the dual emission nature of these systems, which generate both gravitational waves and electromagnetic (EM) radiation. Understanding these processes is imperative for developing predictive models that can assist observational efforts aimed at detecting such phenomena.

Central to the paper is the implementation of a new three-dimensional general relativistic magnetohydrodynamics (GRMHD) framework. This innovative model integrates multiple numerical codes for simulating the inspiral and merger of equal-mass, non-spinning supermassive black hole binaries—starting with realistic initial data and continuing through to the merger. An approximation for gas cooling, through photon emission, is employed throughout the simulations—a feature critical in capturing the thermal dynamics of these astrophysical systems.

Key Results

The research employs this computational mode to track the evolution of black hole binaries over approximately 200 orbits. The paper reveals several critical findings: as the binary shrinks, the distribution of gas changes dramatically. Large-scale, coherent minidisks transform into dispersed accretion flows due to tidal disruption forces as the binary squeezes closer during the inspiral phase. Notably, during the process called "decoupling"—where the black hole binary's orbital evolution occurs too rapidly for the circumbinary disk to respond—the total accretion rate onto the binary slightly declines, yet remains approximately constant over a significant duration close to the merger. This raises essential insights into the understanding of accretion flows near such close-proximity binaries.

Furthermore, significant EM emissions are noted, including an increase by ∼50% at the merger, suggesting a marked change in the EM spectrum tied to abrupt shifts in accretion flow structures. The post-merger system displays a notable alteration in Poynting-flux as the magnetic flux gathered at the merger remnant's spin-parameter (~0.68) facilitates a relativistic jet formation. These Poynting-flux-dominated jets represent the potential for such objects to generate a range of signals detectable via EM surveys.

Implications

The implications of these results are substantial both practically and theoretically in astrophysics. Practically, this paper enhances the predictive models necessary for cross-referencing EM observations with gravitational wave detections, potentially offering a more comprehensive temporal picture of black hole mergers. Theoretically, the interplay between binary dynamics and accretion physics, especially near the merger, offers a deeper understanding of disk morphology transitions driven by relativistic effects and magnetic fields.

Future Developments

Looking ahead, this field of research can expand in several directions. Increasing the complexity of the models to include various spin parameters, field geometries, and mass ratios would provide insights into a broader class of supermassive black hole binaries. Incorporating additional physical processes like radiation transport and pair production could refine the understanding of EM emissions. Finally, extrapolating these findings to explore potential observational signatures—they could be pivotal in the forthcoming era of multi-messenger astronomy.

Overall, this paper demonstrates commendable progress in bridging the gap between computational predictions and observational potentialities, paving the way for future innovations capable of capturing the complexity and grandeur of merging supermassive black holes.