- The paper demonstrates that gas squeezing during SMBH mergers can trigger accretion rates up to 72 times the Eddington limit.
- It employs 3D SPH simulations to capture detailed tidal interactions and shows that less than 5% of the disc mass is expelled.
- The findings suggest that the accretion spike may generate a detectable electromagnetic precursor to gravitational wave events.
Gas Squeezing During the Merger of a Supermassive Black Hole Binary
This paper explores the phenomenon of gas accretion during the merger of a supermassive black hole binary (SMBHB), with particular focus on the dynamics when the binary is embedded within a circumprimary accretion disc. Using three-dimensional simulations conducted with the Phantom Smoothed Particle Hydrodynamics (SPH) code, the authors investigate the accretion rates during the gravitational wave-driven merger and provide evidence for a "squeezing phenomenon" that occurs within the inner disc gas as the secondary black hole spirals inward toward the primary.
Key Findings and Numerical Results
Contrary to previous studies which suggested limited accretion activity during the final stages of a SMBHB merger, this research finds that the inner disc gas experiences significant compression as the secondary black hole approaches the primary. This compression results in an enhancement of accretion rates that exceed the Eddington limit—by factors up to 72 in the simulations—indicating super-Eddington accretion is possible in the final stages. This accretion is facilitated by the enhanced tidal interactions with the secondary black hole.
The simulations, under various conditions, reveal that the mass expelled from the secondary's orbit is negligible, accounting for less than 5% of the initial disc mass. This is a significant divergence from earlier two-dimensional simulations that indicated higher mass ejection when thicker discs were analyzed. These results are pivotal as they suggest the possibility of a luminous electromagnetic precursor that could be detected ahead of the gravitational wave signal from the coalescence of the binary, presenting opportunities for multi-messenger astrophysics.
Implications and Future Directions
From a theoretical perspective, the results challenge previous models of SMBHB merger dynamics that suggested minimal accretion and absence of super-Eddington activity. The novel insight into the squeezing mechanism provides a framework for understanding the coupling between black hole dynamics and surrounding gas in extreme gravitational environments. The findings emphasize the importance of disc thickness, with thinner discs being more efficient in facilitating accretion spikes due to squeezing.
Practically, the potential for detecting super-Eddington luminosity bursts prior to gravitational wave events opens new observational possibilities. This research underscores the need for coordinated observation strategies involving both electromagnetic and gravitational wave observatories. Future work should extend the parameter space, particularly exploring different mass ratios, varying disc properties, and including relativistic effects, to gain a more comprehensive understanding of the conditions under which electromagnetic precursors might be detectable.
In conclusion, this paper contributes significant insights into the accretion dynamics during SMBHB mergers and suggests new avenues for empirical investigation, potentially advancing our ability to identify and characterize such cosmic events through both gravitational and electromagnetic signatures.