Comprehensive analysis of time-domain overlapping gravitational wave transients: A lensing study

Abstract

Next-generation gravitational-wave (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 ℳB/ℳA ∈{0.5,1,2}, signal-to-noise ratios (SNRs) SNRB/SNRA ∈{0.5,1}, and coalescence-time offsets Δ⁢𝑡c ∈[−0.1,0.1] s, and extend to a population analysis. Bayesian PE and fitting-factor studies show that the Type-II lensing hypothesis is favored over the unlensed quasicircular hypothesis (log10⁡ℬLU >1) only in a small region of the overlapping parameter space with ℳB/ℳA ≳1 and |Δ⁢𝑡c| ≤0.03 s, with the inferred Morse index clustering near 𝑛𝑗 ≃0.5, indicative of Type-II lensing, for the cumulative study. 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 |Δ⁢𝑡c|. The microlensing hypothesis is maximally favored (log10⁡ℬLU ≫1) for ℳB/ℳA ≳1 and equal SNRs, increasing with |Δ⁢𝑡c|. The inferred redshifted lens masses lie in the range 𝑀𝑧L∼102–105⁢𝑀⊙ with impact parameters 𝑦∼0.1–3 RE. 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.