Can GW231123 have a stellar origin?

ABSTRACT The gravitational wave event GW231123 detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) interferometers during their fourth observing run features two black holes (BHs) with source-frame masses of $137^{+23}_{-18}$ and $101^{+22}_{-50}\,{\rm M}_\odot$ – 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 ($\chi _1 \simeq 0.9$, $\chi _2 \simeq 0.8$). The primary object in GW231123 is the heaviest stellar mass BH detected to date, which, together with its extreme rotation, raises questions about its astrophysical origin. Accounting for the unusually large spin of ${\sim} 0.9$ with hierarchical mergers requires some degree of fine-tuning. We investigate whether such a massive, highly spinning object could plausibly form from the collapse of a single rotating massive star. We simulate stars with an initial core mass of $160\, {\rm M}_\odot$ – sufficient to produce BH masses at the upper edge of the 90 per cent credible interval for $m_1$ in GW231123 – across a range of rotation rates and $^{12}\mathrm{C}(\alpha ,\gamma)^{16}\mathrm{O}$ reaction rates. We allow for differential rotation to explore the high-spin regime. 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 gravitational wave predictions, and (ii) highly spinning BHs with masses ${\gtrsim} 150 \,\rm M_\odot$ 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 photodisintegration instability.