Is GW231123 a Hierarchical Merger?

Abstract The binary black hole merger GW231123 is both the most massive gravitational-wave event observed and has the highest component spins measured to date. The dimensionless spins of the more massive (primary) and less massive (secondary) black holes are measured to be χ 1 = 0.9 0 − 0.19 + 0.10 and χ 2 = 0.8 0 − 0.51 + 0.20 (90% credible intervals), respectively. Its large mass and extremal spins are challenging to explain through standard binary stellar physics, though a flurry of hypothetical scenarios have been proposed. Hierarchical assembly—i.e., mergers of black holes that are themselves formed from previous generations of mergers—is generally a promising way to explain massive and rapidly spinning black holes. Here, we investigate the possibility that GW231123 was assembled hierarchically in a dense star cluster as the merger of two second-generation black holes. Taking the inferred spin values at face value, we find that it is possible ( p ≈ 5%) that a compact binary with component spins like GW231123 could form in a cluster from hierarchical assembly.