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

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.