Can Eccentric Binary Black Hole Signals Mimic Gravitational-Wave Microlensing?

TL;DR

This study investigates how eccentric binary black hole signals can mimic gravitational-wave microlensing effects, revealing degeneracies that could lead to false detections and emphasizing the importance of including eccentricity in waveform models.

Contribution

It demonstrates the degeneracy between eccentric BBH signals and microlensing signatures and shows that incorporating eccentric waveforms can resolve this ambiguity.

Findings

High eccentricity and low mass binaries can be misclassified as microlensed signals.

Waveform models including eccentricity eliminate degeneracy and correctly identify unlensed signals.

Strong Bayesian evidence for microlensing may be a false positive if eccentricity is not considered.

Abstract

Gravitational lensing in the wave-optics regime imprints characteristic frequency-dependent amplitude and phase modulations on gravitational-wave (GW) signals, yet to be detected by ground-based interferometers. Similar modulations may also arise from orbital eccentricity, raising the possibility of degeneracies that could lead to false microlensing claims. We investigate the extent to which eccentric binary black hole (BBH) signals can mimic microlensing signatures produced by an isolated point-mass lens. With a simulated population of eccentric signals using numerical relativity simulations and \texttt{TEOBResumS-Dal\'i} waveform model, we perform a Bayesian model-comparison study, supported by a complementary \textit{mismatch} analysis. We find a strong degeneracy for high eccentricities, low total masses, and high signal-to-noise ratios (SNRs): under these conditions, quasicircular microlensed model can be strongly favored over quasicircular unlensed model, even when the true signal is unlensed. For moderate SNRs ($\sim 30$), binaries with $M_\mathrm{tot}\lesssim 100\,M_\odot$ and eccentricity $e \gtrsim 0.4$ are particularly susceptible to misclassifications. In such cases, inferred microlens parameters exhibit well-constrained posteriors despite being unphysical. Crucially, the degeneracy is completely removed when the recovery uses waveform models that incorporate eccentricity, which overwhelmingly favors the eccentric hypothesis over microlensing. Our results demonstrate that any event exhibiting strong Bayesian evidence for microlensing should also be analyzed with eccentric waveform models and vice-versa to avoid false positives and biased astrophysical inference. This work contributes to developing robust strategies for interpreting signals in the era of precision GW astronomy.