Comprehensive analysis of time-domain overlapping gravitational wave transients: A Lensing Study

TL;DR

This study investigates how overlapping gravitational wave signals from black hole binaries can mimic gravitational lensing effects, potentially leading to false positives in lensing detection, especially with improved detector sensitivities.

Contribution

The paper provides a detailed analysis of degeneracies between overlapping GW signals and lensing signatures, highlighting conditions that cause false lensing evidence in next-generation detectors.

Findings

Type-II lensing favored only in small parameter regions

Overlapping signals can mimic microlensing time delays

False lensing identifications increase with detector sensitivity

Abstract

Next-generation GW detectors will produce a high rate of temporally overlapping signals from unrelated compact binary coalescences. Such overlaps can bias parameter estimation (PE) and mimic signatures of other physical effects, such as gravitational lensing. In this work, we investigate how overlapping signals can be degenerate with gravitational lensing by focusing on two scenarios: Type-II strong lensing and microlensing by an isolated point-mass lens. We simulate quasicircular binary black-hole pairs with chirp-mass ratios $\mathscr{M}_{\rm B}/\mathscr{M}_{\rm A}\in\{0.5,\,1,\,2\}$, SNR ratios $\mathrm{SNR}_{\rm B}/\mathrm{SNR}_{\rm A}\in\{0.5,\,1\}$, and coalescence-time offsets $\Delta t_{\rm c}\in[-0.1,\,0.1]~\mathrm{s}$. Bayesian PE and fitting-factor studies show that the Type-II lensing hypothesis is favored over the unlensed quasicircular hypothesis ($\log_{10}\mathscr{B}^{\rm L}_{\rm U}>1$) only in a small region of the overlapping parameter space with $\mathscr{M}_{\rm B}/\mathscr{M}_{\rm A}\gtrsim1$ and $|\Delta t_{\rm c}|\leq0.03~\rm{s}$.. Meanwhile, false evidence for microlensing signatures can arise because, to a reasonable approximation, the model produces two superimposed images whose time delay can closely match $|\Delta t_{\rm c}|$. Overall, the inferred Bayes factor depends on relative chirp-mass ratios, relative loudness, difference in coalescence times, and also the absolute SNRs of the overlapping signals. Cumulatively, our results indicate that overlapping black-hole binaries with nearly equal chirp masses and comparable loudness are likely to be falsely identified as lensed. Such misidentifications are expected to become more common as detector sensitivities improve. While our study focuses on ground-based detectors using appropriate detectability thresholds, the findings naturally extend to next-generation GW observatories.