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Determine the spin orientation of GW231123's component black holes

Ascertain the polar tilt angles of both component black hole spins relative to the orbital angular momentum in the gravitational-wave event GW231123 by developing or applying inference methods that resolve the current discrepancies across waveform models and between analyses of LIGO Hanford-only and LIGO Livingston-only data.

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Background

GW231123 is an exceptionally short, high-mass binary black hole signal for which waveform systematics are substantial, particularly at high spin. The authors report large disagreements among state-of-the-art precessing, higher-mode waveform models and note that analyses using data from individual detectors can yield conflicting results.

Because reliable measurement of spin orientations (tilt angles) underpins the interpretation of precession, recoil, and formation channels, resolving these discrepancies is essential to characterizing the source.

References

We are unable to reliably infer the spin orientation of the binary; we infer polar angles between each spin vector and the orbital angular momentum that vary not only between models, but also when independently analysing data obtained by LIGO Livingston compared to LIGO Hanford, see Appendix~\ref{sec:pe_appendix}. When spin misalignment is inferred, we are unable to conclusively constrain the spin orientation away from aligned.

— GW231123: a Binary Black Hole Merger with Total Mass 190-265 $M_{\odot}$ (2507.08219 - Collaboration et al., 10 Jul 2025) in Section 4, Source properties — Subsection Inference