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A new analysis of the gravitational-wave signal GW231123 finds evidence that the colliding black holes were following an eccentric orbit, a result that bolsters a dynamical, hierarchical origin for this unusual event but falls short of a definitive proof.
GW231123 is notable for two reasons. It is the most massive binary black hole merger seen so far and contains a component whose mass lies in the so-called pair-instability mass gap — a range where single-star evolution is not expected to produce black holes. Together with unusually large inferred spins and an unusually short duration in the detectors’ sensitive band, these features have made GW231123 a strong candidate for a hierarchical merger: a system in which at least one component itself formed from an earlier black-hole merger inside a dense stellar environment.
The new study reanalysed LIGO–Virgo data from 11 to 448 Hz using an eccentric, spin-precessing effective-one-body waveform model called TEOBResumS–Dalí and explicitly marginalised over detector calibration uncertainty. The analysts measured an eccentricity of e ≈ 0.34 (+0.12/−0.23) at 5.5 Hz (90% credible interval) and report a Bayes factor of about 10 in favour of the eccentric model over a standard quasi-circular model. In Bayesian model-comparison terms, that is moderate-to-strong support for eccentricity but not an absolute confirmation.
Why does eccentricity matter? Binaries formed in isolation through the standard evolution of two massive stars are expected to have circularised long before they enter the frequency band of ground-based detectors. By contrast, dynamical formation channels in dense environments such as globular clusters or nuclear star clusters can produce binaries that retain measurable orbital eccentricity when they merge. Eccentricity, together with a mass in the pair-instability gap and component spins consistent with being the product of a previous merger, would form a coherent picture pointing to a hierarchical dynamical origin.
The paper also shows that including eccentricity changes the inferred spins. In previous quasi-circular analyses GW231123 showed a preference for a near-extremal spin on the smaller component (very close to the maximum allowed by general relativity). When eccentricity is allowed, that preference weakens, and the posterior median for the secondary spin drops to about a2 ≈ 0.7 — a value close to the characteristic spin expected for a black-hole remnant of an earlier merger. That shift makes the hierarchical interpretation more natural: a remnant black hole with a merger-like spin could have become the heavier component of the new binary.
The authors also carried out injection–recovery tests that indicate analyses starting at 20 Hz — as was common in earlier work — can miss or obscure eccentricity that becomes apparent when lower frequencies down to ~11 Hz are included. This underscores the importance of waveform models and the chosen frequency band when interpreting short-duration, high-mass signals.
Caveats remain. The evidence depends on a particular eccentric, precessing waveform model and on how detector calibration and systematic uncertainties are treated. Waveform modelling of highly dynamical, high-mass mergers is challenging, and alternative models or future improvements could change the quantitative conclusions. The Bayes factor reported is suggestive rather than conclusive, and a single event is never enough to settle formation-channel questions.
If the eccentric interpretation holds up to further scrutiny and is echoed in additional events, it would strengthen the case that dense stellar environments produce hierarchical mergers that can populate the pair-instability mass gap. For now the result is an intriguing step toward a dynamical explanation of GW231123, and a reminder that orbital eccentricity is an important observable for distinguishing how black-hole binaries form.