Detection Prospects for Retrolensing Gravitational Wave Images in Hierarchical Triple Systems

Gravitational wave (GW) retrolensing is a relativistic strong lensing scenario where the GW source is located between the lensing black hole and the observer. Such GW images pass close to the lens and have highly bent trajectories. While such retrolensing images are typically much fainter than the primary images, they become significantly magnified when the GW sources traverse near or through the lensing caustics. These images may then be discernible in future space-based GW observatories such as LISA and DECIGO. In this study, we investigate the detection prospects of retrolensing GW images in nearly edge-on hierarchical triple systems where a stellar-mass binary orbits a supermassive Kerr black hole. Under this scenario, the detection possibility is enhanced since Kerr spacetime admits extended caustics, and the separation between the lens and the source can be significantly smaller than the distance between the triple system and the observer. Our study also complements existing literature on repeated lensing in triple systems.