Can GW231123 have a stellar origin?

TL;DR

This paper explores whether the massive, highly spinning black hole in GW231123 could originate from the collapse of a single massive star, challenging the idea that such objects only form through hierarchical mergers.

Contribution

It demonstrates that rotation can shift the pair-instability mass gap and enable the formation of massive, high-spin black holes from single star collapse, providing a new possible origin for GW231123.

Findings

Rotation shifts the pair-instability mass gap to higher masses.

Highly spinning black holes above 150 solar masses can form from single star collapse.

The primary in GW231123 may be the first observed black hole formed via direct core collapse.

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 ($\chi_1 \simeq 0.9$, $\chi_2 \simeq 0.8$). 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 $160\,M_\odot$ -- 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 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.