First Resolution of Microlensed Images of a Binary-Lens Event

TL;DR

This paper reports the first direct resolution of multiple images in a binary-lens microlensing event using VLTI, enabling detailed characterization of the binary system and demonstrating a new method for studying intermediate-separation stellar binaries.

Contribution

The study presents the first resolution of microlensed images of a binary system with VLTI, providing precise measurements of stellar masses and separation, and opening new avenues for binary system research.

Findings

Resolved binary-lens images with VLTI.

Determined stellar masses and separation.

Demonstrated potential for studying dark compact objects.

Abstract

We resolve the multiple images of the binary-lens microlensing event ASASSN-22av using the GRAVITY instrument of the Very Large Telescope Interferometer (VLTI). The light curves show weak binary-lens perturbations, complicating the analysis, but the joint modeling with the VLTI data breaks several degeneracies, arriving at a strongly favored solution. Thanks to precise measurements of angular Einstein radius \theta_E = 0.724 +/- 0.002 mas and microlens parallax, we determine that the lens system consists of two M dwarfs with masses of M_1 = 0.258 +/- 0.008 M_sun and M_2 = 0.130 +/- 0.007 M_sun, a projected separation of r_\perp = 6.83 +/- 0.31 au and a distance of D_L = 2.29 +/- 0.08 kpc. The successful VLTI observations of ASASSN-22av open up a new path for studying intermediate-separation (i.e., a few astronomical units) stellar-mass binaries, including those containing dark compact objects such as neutron stars and stellar-mass black holes.