Can GW231123 have a stellar origin?

The gravitational wav e ev ent GW231123 det ect ed by the Laser Int erferomet er Gra vitational-Wa v e Observat ory (LIGO) int erferomet ers during their fourth observing run features two black holes (BHs) with source-frame masses of 137+23-18 and 101+22-50 M⊙ –in the range of the pair-instability BH mass gap predicted by standard stellar evolution theory. Both BHs are also inferred to be rapidly spinning ( χ1 ≈ 0.9 , χ2 ≈ 0.8 ). The primary object in GW231123 is the heaviest stellar mass BH det ect ed t o dat e, which, t ogether with its e xtr eme r otation, raises questions about its astr ophysical origin. A ccounting for the unusually large spin of ∼0 . 9 with hierarchical mergers requires some degree of fine-tuning. We inv estigat e whether such a massive, highly spinning object could plausibly form from the collapse of a single rotating massive star . W e simulate stars with an initial core mass of 160 M⊙ – sufficient to produce BH masses at the upper edge of the 90 per cent credible interval for m 1 in G W231123 –acr oss a range of rotation rates and 12C(α, γ) 16O reaction rates. We allow for differential r otation to e xplor e the high-spin r egime. In this limit of weak angular momentum transport, we find that (i) rotation shifts the pair-instability mass gap to higher masses, introducing an important correlation between masses and spins in gr avitational w ave predictions, and (ii) highly spinning BHs with masses ≳ 150 M⊙ can form above the mass gap. Our results suggest that the primary object of GW231123 may be the first directly observed BH that formed via direct core collapse following the phot odisint egration instability.