Polarization Birefringence and Waveform Systematics in GW231123

Abstract

GW231123 is a short, massive binary-black-hole event whose source properties show strong waveform dependence. We use this event to test gravitational-wave polarization birefringence, modeled as a frequency-dependent rotation of the tensor-polarization basis. Instead of sampling a distance-normalized coefficient directly, we sample the band-differential rotation $δ_{\rm br}=Δ(448\,\mathrm{Hz})-Δ(20\,\mathrm{Hz})$ with prior $[-π,π]$, and report the derived coefficient $β_{\rm br}^{\rm derived}$ for comparison with standard propagation parametrizations. We analyze three waveform families: IMRPhenomXPHM (XPHM), IMRPhenomXO4a (XO4a), and NRSur7dq4. The derived posteriors are consistent with the general relativity value, giving $90\%$ upper limits $|β_{\rm br}^{\rm derived}|_{90}=0.378,\,0.097,\,0.273$ for XPHM, XO4a, and NRSur7dq4, respectively. The directly sampled $δ_{\rm br}$ posterior remains broad, with $|δ_{\rm br}|_{90}\simeq2.8\,\mathrm{rad}$, so the accumulated rotation across the analysis band is weakly constrained. The Bayes factors are waveform dependent: $\ln\mathcal{B}_{\rm br/GR}=-1.26\pm0.30$, $+3.64\pm0.28$, and $-0.86\pm0.29$, respectively. We therefore find no waveform-robust evidence for parity-violating propagation. The positive XO4a result is better interpreted as a waveform-dependent birefringence-like response associated with the mass-ratio--distance--spin degeneracy of this short high-mass event.