Accretion is All You Need: Black Hole Spin Alignment in Merger GW231123 Indicates Accretion Pathway

GW231123 represents the most massive binary-black-hole merger detected to date, lying firmly within, or even above, the pair-instability mass gap. The component spins are both exceptionally high ($a_1 = 0.90^{+0.10}{-0.19}$, $a_2 = 0.80^{+0.20}{-0.51}$), which is difficult to explain with repeated mergers. Here we show that the black hole spin vectors are closely aligned with each other while significantly tilted relative to the binary’s orbital angular momentum, pointing to a common accretion-driven origin. We examine astrophysical formation channels capable of producing near-equal, high-mass, and mutually aligned spins consistent with GW231123 – particularly binaries embedded in AGN disks and Pop~III remnants, which grew via coherent misaligned gas accretion. We further argue that other high-mass, high-spin events, e.g., GW190521 may share a similar evolutionary pathway. These findings underscore the critical role of sustained, coherent accretion in shaping the most extreme black hole binaries.

💡 Research Summary

This paper presents a detailed analysis of the gravitational-wave event GW231123, the most massive binary black hole merger detected to date, with component masses of approximately 137 and 103 solar masses. The event lies within or above the theoretical pair-instability mass gap. A key finding is the first statistically significant measurement of the relative spin angle (θ12) between the two black holes. The analysis reveals that the spin vectors of the two black holes are strongly aligned with each other, while simultaneously being significantly misaligned with the binary’s orbital angular momentum.

The authors argue that this specific spin geometry—mutual alignment combined with orbital misalignment—poses a challenge for hierarchical merger scenarios, where independently formed black holes are expected to have uncorrelated spin orientations. Instead, the pattern strongly points towards a common formation and evolution pathway driven by sustained, coherent gas accretion. In such a scenario, both black holes evolve within a shared environment (like an AGN disk or a dense primordial gas cloud), where accretion flows impart angular momentum, aligning their spins with each other over time. The significant tilt relative to the orbit suggests the accretion flow itself was misaligned with the binary plane, possibly due to thick disks or polar accretion streams.

The paper further consolidates the accretion hypothesis by examining other properties of GW231123. The extreme masses can be explained by Eddington or super-Eddington accretion over millions of years. The mass ratio (q ~ 0.75) is consistent with accretion’s tendency to preferentially feed the secondary, driving mass ratios toward unity. The high spin magnitudes (χ ~ 0.9) sit at the empirical limit achievable through prolonged disk accretion, whereas reproducing such high spins via hierarchical mergers requires fine-tuned conditions.

The authors discuss two primary astrophysical environments capable of hosting such accretion-driven evolution: the gaseous disks around active galactic nuclei (AGNs) and the dense remnants of metal-free Population III stars. They compare timescales, showing that for binaries formed at sufficiently wide separations, there is ample time for accretion to spin up the black holes before gravitational-wave emission drives them to merger. Finally, they suggest that other high-mass, high-spin events like GW190521 may share a similar accretion-driven origin. The study underscores that sustained accretion is not merely a mass-growth mechanism but a fundamental process shaping the spin architecture of the most extreme black hole binaries observable with gravitational waves.