Can GW231123 have a stellar origin?

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 $137^{+23}_{-18} M_\odot$ and $101^{+22}_{-50} M_\odot$ -- in the range of the pair-instability black hole mass gap predicted by standard stellar evolution theory. Both black holes are also inferred to be rapidly spinning ($χ_1 \simeq 0.9$, $χ_2 \simeq 0.8$). 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 $160\,M_\odot$ -- 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 $^{12}\mathrm{C}(α,γ)^{16}\mathrm{O}$ 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 black hole that formed via direct core collapse following the photodisintegration instability.

Figures (3)

• Figure 1: Final BH mass as a function of initial rotation with initial helium core mass $160 \rm {\rm M}_\odot$, for several values of the $^{12}\mathrm{C}(\alpha,\gamma)^{16}\mathrm{O}$ rate. The top panel shows results as a function of the initial stellar rotation $\Omega$ in units of the critical value $\Omega_{\rm crit}$; the bottom panel shows results as a function of the BH spin $\chi$. Stars with faster rotation and smaller $^{12}\mathrm{C}(\alpha,\gamma)^{16}\mathrm{O}$ reaction rate undergo PISN and are not shown in the figure. The shaded contours indicate the $90\%$ credible intervals of the primary mass (orange) and spin (purple) in GW231123 LIGOScientific:2025rsn.
• Figure 2: Central temperature $T_c$ versus time since core helium depletion for the simulations described in text, with $M_{\rm in} = 160 {\rm M}_\odot$ for the median $^{12}\mathrm{C}(\alpha,\gamma)^{16}\mathrm{O}$ rate. Non-rotating models reach higher core densities and temperatures, crossing into the regime where photodisintegration reactions lead to gravitational collapse, here assumed to be $T_c = 9\times 10^9 \rm\, K$. Rotating models, by contrast, terminate at lower $T_c$, thereby avoiding collapse and instead undergoing a PISN.
• Figure 3: Final black‐hole mass (top) and dimensionless spin parameter $\chi$ (bottom) as functions of the initial rotation rate $\Omega/\Omega_{\rm crit}$ for two choices of the prompt‐collapse core mass threshold, $M_{\rm cutoff}=3\,M_\odot$ and $5\,M_\odot$.