Detecting binarity of GW150914-like lenses in gravitational microlensing events

TL;DR

This paper investigates the ability to distinguish binary black hole lenses from single lenses in gravitational microlensing events, demonstrating that microlensing can detect wider BBH separations than gravitational wave observations.

Contribution

It introduces a Bayesian analysis method to identify binary black hole lenses in microlensing data, expanding the potential for discovering BBHs with wider separations.

Findings

Binarity detectable for BBH separations down to 0.025 Einstein radii

Microlensing sensitive to wider BBH separations than GW methods

Potential to discover BBH populations from different formation channels

Abstract

The recent discovery of gravitational waves (GWs) from stellar-mass binary black holes (BBHs) provided direct evidence of the existence of these systems. BBH lenses would have gravitational microlensing signatures that are distinct from single-lens signals. We apply Bayesian statistics to examine the distinguishability of BBH microlensing events from single-lens events under ideal observing conditions, using the photometric capabilities of the Korean Microlensing Telescope Network. Given one year of observations, a source star at the Galactic Centre, a GW150914-like BBH lens (total mass 65M$_\odot$, mass ratio 0.8) at half that distance, and an impact parameter of 0.4 Einstein radii, we find that binarity is detectable for BBHs with separations down to 0.0250 Einstein radii, which is nearly 3.5 times greater than the maximum separation for which such BBHs would merge within the age of the Universe. Microlensing searches are thus sensitive to more widely separated BBHs than GW searches, perhaps allowing the discovery of BBH populations produced in different channels of binary formation.