Stellar-mass microlensing of gravitational waves

TL;DR

This paper investigates how stellar-mass objects can microlens gravitational waves, affecting their waveforms and magnification, with implications for detection and cosmological studies, emphasizing the importance of diffraction effects and lens environment.

Contribution

It demonstrates the significance of diffraction effects in stellar-mass microlensing of gravitational waves and highlights the impact of the host galaxy environment on lensing configurations.

Findings

Diffraction effects are crucial for GWs lensed by objects with masses ≤ 100 M_7.

Galactic environment alters microlensing configurations, preventing isolated point-mass approximations.

Stellar lenses (~1 M_7) significantly suppress magnification, simplifying the lensing environment.

Abstract

When gravitational waves pass through the nuclear star clusters of galactic lenses, they may be microlensed by the stars. Such microlensing can cause potentially observable beating patterns on the waveform due to waveform superposition and magnify the signal. On the one hand, the beating patterns and magnification could lead to the first detection of a microlensed gravitational wave. On the other hand, microlensing introduces a systematic error in strong lensing use-cases, such as localization and cosmography studies. We show that diffraction effects are important when we consider GWs in the LIGO frequency band lensed by objects with masses $\lesssim 100 \, \rm M_\odot$. We also show that the galaxy hosting the microlenses changes the lensing configuration qualitatively, so we cannot treat the microlenses as isolated point mass lenses when strong lensing is involved. We find that for stellar lenses with masses $\sim 1 \, \rm M_\odot$, diffraction effects significantly suppress the microlensing magnification. Thus, our results suggest that gravitational waves lensed by typical galaxy or galaxy cluster lenses may offer a relatively clean environment to study the lens system, free of contamination by stellar lenses. We discuss potential implications for the strong lensing science case. More complicated microlensing configurations will require further study.