Is GW231123 a hierarchical merger?

TL;DR

GW231123, a notably massive and highly spinning BBH merger, challenges standard stellar-collapse formation due to the inferred masses and spins $m_1\approx 137$ and $m_2\approx 103$ with $χ_1\approx0.90$ and $χ_2\approx0.80$. The authors test whether a 2G+2G hierarchical merger in a globular cluster can reproduce these spins by applying the population-test framework of passengerAreAllModels2024 and using a cluster-based remnant-spin distribution with natal spins $χ_{birth}=0.2$, marginalized over cluster escape velocity. They analyze two waveform calibrations, XO4a and NRSur, and perform injections to compute a normalised evidence $Z̄$ and a p-value; they find $p<0.01$ for XO4a and $p<0.01$ for NRSur, disfavouring the 2G+2G channel under both waveforms. Overall, the study constrains hierarchical-merger scenarios for this event and motivates considering alternative channels (e.g., 2G+1G or 3G+1G) or different environments, while highlighting sensitivities to waveform systematics and detector noise in high-spin, high-mass events.

Abstract

The binary black hole merger GW231123 is both the most massive gravitational-wave event observed and has the highest component spins measured to date. The dimensionless spins of the more massive (primary) and less massive (secondary) black holes are measured to be $χ_1 = 0.90^{+0.10}_{-0.19}$ and $χ_2 = 0.80^{+0.20}_{-0.51}$ ($90\%$ credible intervals), respectively. Its large mass and extremal spins are challenging to explain through standard binary stellar physics, though a flurry of hypothetical scenarios have been proposed. Hierarchical assembly -- i.e., mergers of black holes that are themselves formed from previous generations of mergers -- is generally a promising way to explain massive and rapidly spinning black holes. Here, we investigate the possibility that both GW231123 was assembled hierarchically in a dense star cluster as the merger of two second-generation black holes. Taking the inferred spin values at face value, we find that it is possible, though unlikely ($p\lesssim 1\%$), that a compact binary with both component spins like GW231123 could form in a cluster from hierarchical assembly.

Figures (2)

• Figure 1: Comparing the predicted distribution of black hole spins in globular clusters to the posteriors for $\chi_1, \chi_2$ in GW231123. In solid blue we plot the posterior for the IMRPhenomXO4a waveform model and in dashed orange we plot the posterior for the NRSur7dq4 waveform model. The three curves represent, one-, two-, and three-sigma intervals. The dotted curves show the distribution expected for globular clusters; $\chi_\text{birth} = 0.2$ in black and $\chi_\text{birth} = 0.02$ in red. These contours are 99% credible intervals. Note that the probability axes in the 1D distributions are plotted with a log scale.
• Figure 2: Distributions of normalized evidence $\overline{\mathcal{Z}}$ from passengerAreAllModels2024. The histograms show the distribution predicted by our cluster model, while the dashed red lines show the value measured for GW231123. For the XO4a analysis, GW231123 is excluded from the injected distribution with $p\ll{1\%}$. For the NRSur analysis, GW231123 is excluded with $p \lesssim 1\%$.