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

TL;DR

GW231123’s exceptional mass and high spins, coupled with a measurable mutual spin alignment, challenge hierarchical-merger explanations and favor sustained, coherent gas accretion as the dominant growth channel. By computing the relative spin angle $\\theta_{12}$ from GW data and comparing with accretion models, the paper shows that accretion in gas-rich environments can produce both near-equal masses and spins that align with each other but remain tilted relative to the orbital axis. The work contrasts AGN-disk accretion and Pop III remnants, highlighting distinct spin–orbit dynamics, mass–spin trends, and potential eccentricity signatures as discriminants. It also provides concrete observational diagnostics to differentiate these channels and calls for further 3D hydrodynamic simulations to refine gas-flow geometry predictions for future mergers.

Abstract

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.

Figures (2)

• Figure 1: Posterior distribution of the relative spin angle $\theta_{12}$ for GW231123 (red histogram) and GW190521 (gray histogram), compared to theoretical expectations. The dashed black line shows the isotropic prior, $p(\theta_{12}) = \frac{1}{2} \sin \theta_{12}$. The dotted black line shows the distribution expected for perfectly aligned spins observed with Gaussian tilt uncertainty of $\sigma = 45^\circ$. The blue curve shows the distribution obtained by randomly sampling the azimuthal angle $\Delta\phi$ while preserving the measured tilt angles from GW231123. Also shown are the three model distributions used to obtain the GW231123 posterior.
• Figure 2: Gravitational-wave merger time $t_{\rm merge}$ for a GW231123-like binary as a function of its semi-major axis $a$ for eccentricities $e = 0, 0.3, 0.7$ (black to light gray curves). Horizontal dashed lines show the spin-up times $t_{\rm spin}$ required to reach $\chi \simeq 0.9$ for Eddington ratios $f_{\rm Edd}=0.1,1,10$. The markers indicate the separations at which $t_{\rm merge} = t_{\rm spin}$, defining a minimum semi-major axis above which gas accretion has sufficient time to spin the black holes up before gravitational-wave dominated inspiral.