Eccentricity in Disguise? Insights from GW231123 and Numerically Simulated Binary Black Hole Merger Signals

Abstract: GW231123 is a gravitational-wave signal originating from the merger of a black hole binary with total mass $\sim 250 M_{\odot}$, the largest ever detected by the LIGO-Virgo-Kagra Collaboration. Remarkably, under standard priors, the system features among the fastest-spinning binary components confidently measured in binary mergers, $ χ_{1,2} \gtrsim 0.7$ at $90\%$ one-dimensional credibility, according to the most accurate model employed. As typical binary mergers result in remnants with $χ\sim 0.7$, such spin values are challenging to obtain even from previous (hierarchical) mergers. These inferred properties rely on waveform models lacking eccentric corrections in the merger-ringdown stage. Here, we show that binaries retaining significant eccentricity up to merger can be misinterpreted as near-extremally spinning when non-circular corrections are neglected. Binary-agnostic ringdown analysis instead provides unbiased estimates of the remnant properties, provided that a robust estimate of the signal peak can be obtained. We re-analyse GW231123 using available eccentric numerical-relativity catalogues, finding that although eccentric templates can provide a good fit to the data, quasi-spherical templates are still favoured. Ringdown analyses confirm a secondary likelihood peak correlated with large eccentricity values, but improved eccentric models will be required to assess the reliability of this interpretation. Finally, analysing GW231123 under population-informed parametric priors confirms the exceptional nature of this event within the current black hole binary population.