Stellar-mass microlensing of gravitational waves

Abstract

When gravitational waves (GWs) 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 GW. On the other hand, microlensing introduces a systematic error in strong lensing use-cases, such as localization and cosmography studies. By numerically solving the lensing diffraction integral, we show that diffraction effects are important when we consider GWs in the LIGO frequency band lensed by objects with masses ≲ 100M☉. 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 ∼1M☉, diffraction effects significantly suppress the microlensing magnification. Thus, our results suggest that GWs 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, which can be advantageous for localization and cosmography studies.