# Observational signatures of microlensing in gravitational waves at LIGO/Virgo frequencies

## TL;DR

This paper investigates how a realistic population of stellar microlenses embedded in macromodels can produce detectable interference effects in gravitational waves observed by LIGO/Virgo, especially near critical curves and caustics.

## Contribution

It is the first study to analyze the combined effect of microlenses and macromodels on strongly lensed gravitational waves at LIGO/Virgo frequencies.

## Key findings

- Microlenses near critical curves can cause interference distortions.
- Negative parity images near caustics are more likely to show measurable effects.
- Realistic microlens populations can produce observable interference in lensed gravitational waves.

## Abstract

Microlenses with typical stellar masses (a few ${\rm M}_{\odot}$) have traditionally been disregarded as potential sources of gravitational lensing effects at LIGO/Virgo frequencies, since the time delays are often much smaller than the inverse of the frequencies probed by LIGO/Virgo, resulting in negligible interference effects at LIGO/Virgo frequencies. While this is true for isolated microlenses in this mass regime, we show how, under certain circumstances and for realistic scenarios, a population of microlenses (for instance stars and remnants from a galaxy halo or from the intracluster medium) embedded in a macromodel potential (galaxy or cluster) can conspire together to produce time delays of order one millisecond which would produce significant interference distortions in the observed strains. At sufficiently large magnification factors (of several hundred), microlensing effects should be common in gravitationally lensed gravitational waves. We explore the regime where the predicted signal falls in the frequency range probed by LIGO/Virgo. We find that stellar mass microlenses, permeating the lens plane, and near critical curves, can introduce interference distortions in strongly lensed gravitational waves. For those lensed events with negative parity, (or saddle points, never studied before in the context of gravitational waves), and that take place near caustics of macromodels, they are more likely to produce measurable interference effects at LIGO/Virgo frequencies. This is the first study that explores the effect of a realistic population of microlenses, plus a macromodel, on strongly lensed gravitational waves.