In April 2019 the European Space Agency’s Gaia spacecraft identified a brightening object in the Milky Way. The source, dubbed Gaia19bld, turned out to be a gravitational lens that was magnifying the light of a distant star. In the past, astronomers have struggled to characterize such microlenses as Gaia19bld because the lensed images they form are on the scale of milliarcseconds, unlike the much larger lensed images produced by galaxies (see the article by Leon Koopmans and Roger Blandford, Physics Today, June 2004, page 45).

The mass of a microlens is directly proportional to the angular separation between the lensed arcs of light. Because individual ground-based telescopes do not have the resolving power to provide that measurement, generally astronomers have been forced to infer the mass by carefully analyzing small magnification peaks in the photometric data that can be correlated with the size of the lensed object.

Arnaud Cassan of Sorbonne University in France and colleagues set out to directly resolve the lensed light of Gaia19bld. To do so, they spent three nights in July 2019 at the Very Large Telescope observatory in Chile. Pointing four 1.8 m telescopes at Gaia19bld, they used the PIONIER instrument to combine the collected near-IR light interferometrically. The use of interferometry, in effect, created a single, larger telescope of up to 128 m for some observations. The setup enabled sufficient resolution not only to obtain images of the arcs, the first ever derived for a subgalactic lensed object, but also to track the arcs’ rotation about Gaia19bld because of the continuously evolving alignment of the lens and the background star (see figure). The researchers’ determination of the arcs’ separation, along with the parallax to the microlens, led them to a Gaia19bld mass measurement of 1.147 ± 0.029 solar masses.

Characterizing a microlens allows astronomers to learn about both the lens and the source object it magnifies. In a separate study, the researchers conclude that the distant object is a red giant star in the galactic disk located about 8500 parsecs from Earth. The nature of Gaia19bld is trickier to determine because a star of its mass and distance (roughly 5500 parsecs) would not be bright enough to be spotted by telescopes. The researchers suspect it’s a main-sequence star only because 1-solar-mass white dwarfs and neutron stars are both rare.

The fact that the technique does not depend on the brightness of the lens is why it’s so intriguing to astronomers—they should be able to identify otherwise undetectable white dwarfs, neutron stars, and stellar-mass black holes in our galaxy. Telescopes currently being designed to survey the entire sky swiftly and precisely, such as the Vera C. Rubin Observatory in Chile, should be able to detect thousands of microlensing events annually. (A. Cassan et al., Nat. Astron., 2021, doi-10.1038/s41550-021-01514-w.)