Spitzer + VLTI-GRAVITY Measure the Lens Mass of a Nearby Microlensing Event

TL;DR

This study combines space-based, ground-based, and interferometric observations to precisely measure the mass and distance of a nearby lensing star, demonstrating a novel method for studying isolated objects like black holes.

Contribution

It introduces a new approach using interferometry and satellite parallax to unambiguously determine the mass of a microlensing lens, including stellar remnants.

Findings

Lens mass: 0.495 solar masses

Distance to lens: 429 parsecs

First unambiguous interferometry-based mass measurement

Abstract

We report the lens mass and distance measurements of the nearby microlensing event TCP J05074264+2447555. We measure the microlens parallax vector ${\pi}_{\rm E}$ using Spitzer and ground-based light curves with constraints on the direction of lens-source relative proper motion derived from Very Large Telescope Interferometer (VLTI) GRAVITY observations. Combining this ${\pi}_{\rm E}$ determination with the angular Einstein radius $\theta_{\rm E}$ measured by VLTI GRAVITY observations, we find that the lens is a star with mass $M_{\rm L} = 0.495 \pm 0.063~M_{\odot}$ at a distance $D_{\rm L} = 429 \pm 21~{\rm pc}$. We find that the blended light basically all comes from the lens. The lens-source proper motion is $\mu_{\rm rel,hel} = 26.55 \pm 0.36~{\rm mas\,yr^{-1}}$, so with currently available adaptive-optics (AO) instruments, the lens and source can be resolved in 2021. This is the first microlensing event whose lens mass is unambiguously measured by interferometry + satellite parallax observations, which opens a new window for mass measurements of isolated objects such as stellar-mass black holes.