Can stellar physics explain GW231123?

Abstract: The gravitational wave event GW231123 detected by the LIGO interferometers during their fourth observing run features two black holes with source-frame masses of $137^{+22}_{-17} M_\odot$ and $103^{+20}_{-52} M_\odot $ -- well within or above the pair-instability black hole mass gap predicted by standard stellar evolution theory. Both black holes 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 black hole 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% 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 find that: (i) rotation shifts the pair-instability mass gap to higher masses, introducing a significant ingredient that correlates 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, implying that stellar evolution alone is sufficient to explain GW231123. Our results suggest that the primary object of GW231123 may be the first directly observed black hole that formed via direct core collapse following the photodisintegration instability.