Polarization Birefringence and Waveform Systematics in GW231123

TL;DR

This paper tests gravitational-wave polarization birefringence using GW231123, finding results consistent with general relativity and highlighting waveform dependence and weak constraints on accumulated rotation.

Contribution

It introduces a novel approach to measure frequency-dependent polarization rotation and compares multiple waveform models to assess birefringence in gravitational waves.

Findings

Posteriors align with general relativity expectations.

Waveform dependence affects Bayes factors and interpretations.

Weak constraints on the total polarization rotation across the analysis band.

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 $\delta_{\rm br}=\Delta(448\,\mathrm{Hz})-\Delta(20\,\mathrm{Hz})$ with prior $[-\pi,\pi]$, and report the derived coefficient $\beta_{\rm br}^{\rm derived}$ 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 $|\beta_{\rm br}^{\rm derived}|_{90}=0.378,\,0.097,\,0.273$ for XPHM, XO4a, and NRSur7dq4, respectively. The directly sampled $\delta_{\rm br}$ posterior remains broad, with $|\delta_{\rm br}|_{90}\simeq2.8\,\mathrm{rad}$, 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.