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arXiv · 2511.23110

Strong-field Gravitational Wave Lensing in the Kerr Background

M. V. S. Saketh·Rajes Ghosh·Anuj Mishra
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Abstract

Gravitational-wave (GW) lensing can encode valuable information about the properties of the intervening lens, but most existing studies remain restricted to the small-deflection, weak-field regime. To bridge this crucial gap, this work presents the first systematic analysis of strong-field, wave-optical GW lensing by a Kerr black hole (BH), extending recent results for non-rotating lens to the astrophysically more relevant case of spinning-lens. Using the Mano-Suzuki-Takasugi formalism, we compute the strong-field scattering factor and show that the the spin produces characteristic modifications to the lensed waveform, and high-frequency incident radiation is not strongly absorbed by the BH lens, contrary to earlier claims. We further derive explicit expressions for the observed waveform for the general source-lens-observer configuration, showcasing the distortions produced by the scattering and quantifying their departure from the Schwarzschild case. Specializing to on-axis scattering, a mismatch analysis for a GW150914-like source lensed by a Kerr BH of mass $M=10^2~\mathrm{M}_\odot$ situated $100M$ away from the source reveals percent-level deviations from the unscattered wave at scattering angles near $30^\circ$, across a range of lens spin values. The mismatch generally decreases as the scattering angle increases, but this behavior can change substantially when polarization mixing induced by scattering becomes significant. In such cases, components that are absent/suppressed in the direct signal may become appreciable due to scattering effects. For a fixed scattering angle, however, the mismatch shows only a weak dependence on the BH spin in the case of on-axis scattering, which may improve for more general configurations. The framework developed here offers a unified treatment of strong-field GW scattering in Kerr spacetime for interpreting future high-precision GW observations.

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M. V. S. Saketh, Rajes Ghosh, Anuj Mishra. 2025-11-28. Strong-field Gravitational Wave Lensing in the Kerr Background. https://arxiv.org/abs/2511.23110

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