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arXiv · 2510.14363

Is GW231123 a hierarchical merger?

Lachlan Passenger·Sharan Banagiri·Eric Thrane·Paul D. Lasky·Angela Borchers·Maya Fishbach·Claire S. Ye
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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 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 ($p\approx 5\%$) that a compact binary with component spins like GW231123 could form in a cluster from hierarchical assembly.

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Lachlan Passenger, Sharan Banagiri, Eric Thrane, Paul D. Lasky, Angela Borchers, Maya Fishbach, Claire S. Ye. 2026-03-27. Is GW231123 a hierarchical merger?. https://arxiv.org/abs/2510.14363

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