Repeated Gravitational Lensing of Gravitational Waves in Hierarchical Black Hole Triples

TL;DR

This paper explores how hierarchical triple systems with a binary black hole and a supermassive black hole can produce repeated gravitational lensing of gravitational waves, potentially detectable by LISA, revealing their origin and testing strong gravity.

Contribution

It demonstrates the possibility of repeated GW lensing events in hierarchical triples with specific orbital periods, linking GW observations to astrophysical origins.

Findings

Repeated lensing events can occur for outer binary periods around one year.

LISA could detect periodic GW amplitude spikes from lensing.

Detection would confirm BBH origins in nuclear star clusters.

Abstract

We consider binary black holes (BBHs) in a hierarchical triple system where a more compact, less-massive binary is emitting detectable gravitational waves (GWs), and the tertiary is a supermassive BH at the center of a nuclear star cluster. As previous works have shown, the orbital motion of the outer binary can generate a detectable relativistic Doppler boost of the GWs emitted by the orbiting inner binary. We show here that for outer-binary orbits with a period of order one year, there can be a non-negligible probability for repeated gravitational lensing of the GWs emitted by the inner binary. Repeating gravitational lensing events could be detected by the LISA observatory as periodic GW amplitude spikes before the BBH enters the LIGO band. Such a detection would confirm the origin of some BBH mergers in nuclear star clusters. GW lensing also offers new testing grounds for strong gravity.