Attributing Waveform Model Discrepancies in GW231123: A Feature-Based Diagnostic and Robust Astrophysical Inference

Abstract: Gravitational wave parameter estimation is susceptible to systematic uncertainties arising from the choice of waveform model, a challenge particularly acute for complex events like GW231123. We present a comprehensive, data-driven framework to systematically quantify, attribute, and mitigate these model-dependent discrepancies, aiming for more robust astrophysical inferences. Using posterior distributions for GW231123 derived from five distinct waveform models, we quantified discrepancies at both parameter-specific (Jensen-Shannon divergence) and global (Sliced Wasserstein Distance, UMAP) scales. Our core innovation is a feature-based diagnostic that correlates observed discrepancies with intrinsic model characteristics such as domain, calibration method, and treatment of precession or higher-order modes. This analysis revealed significant discrepancies, primarily linked to frequency-domain, phenomenological models (IMRPhenomXPHM and IMRPhenomXO4a), which notably lacked comprehensive higher-order mode or precession physics and exhibited the largest deviations from the numerical relativity surrogate. To provide a robust characterization of the source, we employed Bayesian Model Averaging, weighting each model's contribution by its approximate evidence. This yielded a definitive meta-posterior for GW231123, establishing its primary black hole mass at $134.9^{+24.0}_{-14.6} \, M_{\odot}$ and confirming strong evidence for significant spin-induced precession ($\chi_p = 0.79^{+0.13}_{-0.19}$). The merger formed an intermediate-mass black hole of approximately $221 \, M_{\odot}$. Our findings underscore the critical role of waveform model features in influencing parameter estimates and provide a robust, uncertainty-quantified characterization of GW231123 as a high-mass binary in the pair-instability supernova mass gap, likely formed through dynamical pathways.

Subjects:	astro-ph.CO; cs.LG
Cite as:	PX:2508.00009