The First Model-Independent Upper Bound on Micro-lensing Signature of the Highest Mass Binary Black Hole Event GW231123

TL;DR

This work tests for wave-optics microlensing in GW231123 using a model-independent residual-cross-correlation approach (μ-GLANCE) and a Bayesian amplification framework. Across multiple waveform models, it finds no statistically robust lensing signature, highlighting that waveform systematics in high-mass BBH mergers can masquerade as lensing. The study shows that current waveform uncertainties can significantly bias high-mass parameter inferences and residual features, suggesting caution in claiming lensing for GW231123. Looking ahead, it outlines the potential for future detections and emphasizes the need for improved waveform models and broader-band, next-generation detectors to robustly identify lensed GW signals.

Abstract

The recently discovered gravitational wave (GW) event, GW231123, is the highest mass binary black hole (BBH) merger detected to date by the LIGO-Virgo-KAGRA Collaboration. The inferred source masses of GW231123 lie in a mass range where stellar-progenitor black holes are rare to exist due to the pair instability supernovae mass gap, and hence alternative scenarios of origin of this inferred heavy mass black hole become important. One of such hypotheses of its origin is gravitational lensing that introduces modulations to the amplitude and phase of GWs and can make the inferred mass higher from the true value. In this work, we search for the lensing signatures from GW231123 and all other events in a model-independent approach using the technique \texttt{$μ$-GLANCE} which carries out tests on its residual strain to look for common features across the detector network through cross-correlation and infers the lensing signal in a Bayesian framework. Our analysis tests yield no strong evidence in support for lensing, though it detects presence of potential residual in the data, which can be a micro-lensing signature with a modulation amplitude less than 0.8 at 95\% C.I. However, our study finds that current waveform systematics for such heavy mass binary systems are large enough to shadow the detection of lensing from such short-duration GWs such as GW231123, and hence no concluding claim of lensing could be made at this stage. We conclude that if this event is lensed, then in near future, detection of similar lensed events will take place with current detector sensitivity and hence can open a potential discovery space of lensed GW signal with the aid of more accurate waveform models.

Figures (7)

• Figure 1: In this figure, we show the inference of GW source properties: detected BBH masses ($m_1$ and $m_2$), luminosity distance ($d_{\rm L}$), inclination angle ($\theta_{\rm jn}$), effective-spin ($\chi_{\rm eff}$) and coalescence angle ($\phi$). The results for five different waveform models are shown in different colors. The disagreement between the parameters recovered shows the high systematic uncertainties associated with the waveform modeling for high-mass BBH mergers. Vertical dotted lines show the maximum-likelihood values of these parameters.
• Figure 2: In this figure, along the first column, we show the residuals in detector H1 and L1. In the second column, we show residual cross-correlation for two different timescales $1/8s$ and $1/16s$, to show the variation of the residual cross-correlation with the associated timescale. In the third column we show cumulative residual cross-correlation. Each row is associated with the results given a waveform model, the model name is mentioned in the title of the figure(s). The vertical dashed green and red lines denote the start time and the end time over which the residual cross-correlation SNR is calculated. It is chosen ($1.0s$) over a sufficiently longer timescale than the in-band ($0.2s$) duration, to completely encapsulate the signal. The values of the residual cross-correlation SNR are mentioned in the third panel of each row.
• Figure 3: In this figure, we show the estimation of the lensing parameters for different waveform models. The posteriors obtained for different waveform models agree on the values of the estimated lensing parameters. The estimations of the lensing parameters show consistency with the residual cross-correlation results: higher the residual cross-correlation SNR, higher the amplitude and phase distortions.
• Figure 4: In this figure, we present our findings from the searches of wave-optics lensing features from the GWTC-4 catalog. The horizontal axis shows the events in chronological sequence, and the vertical axis shows the residual SNR. To keep waveform modeling errors in check, we considered only the portion of the waveform when the BBH orbit is larger than the innermost stable circular orbit (ISCO) the frequency of the GW is lower than $f_{\rm isco}$.
• Figure 5: In the figure, we present the recovery of different GW source parameters by a set of waveform models for a simulated heavy-mass ($\mathcal{M}_c = 80 M_{\odot}$, $q=0.75$) signal with IMRPhenomXPHM-SpinTaylor waveform with SNR similar to GW231123 ($\rho_N = 22.78$). The recovery has been performed with a set of three different waveform models: IMRPhenomTPHM, IMRPhenomXO4a and IMRPhenomXPHM-SpinTaylor.