GW231123 is an exceptional gravitational-wave event consistent with the merger of two massive, highly spinning black holes. Reliable inference of the source properties is crucial for accurate interpretation of its astrophysical implications. However, characterization of GW231123 is challenging: only few signal cycles are observed and different signal models result in systematically different parameters. We investigate whether the interpretation of GW231123 is robust against model systematics and Gaussian detector noise. We show that the model systematics observed in GW231123 can be reproduced for a simulated signal based on the numerical-relativity surrogate model nrsur7dq4. Simulating data using the maximum-likelihood nrsur7dq4 waveform for GW231123 and no noise realization, we closely recover the systematics observed for the real signal. We then explore how the headline properties of GW231123 are impacted by Gaussian detector noise. Using the nrsur7dq4 maximum-likelihood waveform and different noise realizations, we consistently find support for large masses, high spin magnitudes (median ${\chi }_1\ge 0.7$), and high spin precession (median ${\chi }_{\mathrm{p}}\ge 0.68$). The spin in the direction of the angular momentum (${\chi }_{\mathrm{eff}}$) fluctuates more. Finally, again comparing to simulated signals, we show that any differences in the GW231123 inference based on each separate detector are not statistically significant. These results show that the properties of GW231123, and most importantly the high mass and high spin magnitudes inferred by nrsur7dq4, are robust.

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• Classical black holes
• Gravitational waves