A new study outlines how to identify the most promising candidates from ground-based microlensing surveys for follow-up with the GRAVITY+ interferometer, potentially enabling routine mass measurements of isolated black holes for the first time.

To date, nearly all confirmed stellar-mass black holes have been found in binary systems, where they reveal themselves by accreting material from a companion star and emitting X-rays. Truly isolated black holes — objects that neither shine nor interact with nearby matter — remain largely hypothetical, despite theoretical predictions that they should be numerous.

Their only observable signature is gravitational microlensing. When a massive, dark object passes almost directly between Earth and a distant background star, its gravity bends and focuses the starlight, temporarily increasing the star’s apparent brightness. Microlensing events are detected regularly in large sky surveys and have been used to identify planets, brown dwarfs and faint stars.

The difficulty is that photometric microlensing data — measurements of how a star’s brightness changes over time — rarely provide enough information to determine the mass of the lensing object. Long-duration events can suggest a more massive lens, but the duration also depends on distance, relative velocity and geometric alignment. Event timescale alone cannot reliably distinguish a black hole from an ordinary star.

In a paper published in The Astrophysical Journal, Chinese astrophysicists and Przemysław Mróz of the University of Warsaw Astronomical Observatory describe how the upcoming GRAVITY+ interferometer could overcome this limitation and how to select the best microlensing events for its follow-up observations.

GRAVITY+ is an upgraded version of the GRAVITY instrument operating at the Very Large Telescope Interferometer at the Paranal Observatory. The interferometer combines light from multiple telescopes to achieve an angular resolution equivalent to that of a telescope with a diameter of more than 100 meters — far exceeding the capability of any single optical telescope.

During a microlensing event, gravity produces two closely spaced images of the background star on opposite sides of the lens. In standard photometric data, these images cannot be resolved; observers see only a temporary brightening. An interferometer such as GRAVITY+, however, can measure the minute positional shifts of the combined light and, in favorable cases, separate the two images. From these geometric measurements, astronomers can determine the angular Einstein radius and, combined with distance information, calculate the mass of the lens directly.

The challenge is efficiency. Large surveys such as the Optical Gravitational Lensing Experiment, conducted at Las Campanas Observatory by the University of Warsaw, detect microlensing events daily. The vast majority are caused by ordinary stars. Interferometric observing time is limited, making it impractical to follow up hundreds of events in search of a few black holes.

The researchers therefore propose a selection criterion based on two measurable quantities from ground-based light curves: event duration and microlensing parallax.

Microlensing parallax arises because Earth moves along its orbit during longer events. This motion introduces subtle asymmetries into the brightness curve, reflecting a change in viewing perspective. Ideally, parallax is measured from widely separated locations, such as Earth and a satellite. The authors show, however, that even ground-based observations can capture a component of the parallax signal that, when combined with event timescale, provides a stronger indication of lens mass.

By constructing a simple test that merges these two observables, the team identifies events more likely to be caused by high-mass, compact objects rather than ordinary stars.

To evaluate the method, the researchers used galactic population simulations and artificial microlensing event catalogues. They estimated how many isolated black holes would be selected under their criteria and how many false positives would remain.

Their results indicate that GRAVITY+ could follow several dozen carefully selected events per year. From those, on the order of a dozen isolated black holes annually could yield direct mass measurements. This would represent a shift from rare, individual detections to a sustained program of mass characterization.

A statistically meaningful sample would allow astronomers to measure the distribution of black hole masses, distances and velocities in the Milky Way. Such data are critical for understanding how massive stars collapse, whether black holes receive significant velocity “kicks” at birth, and how they are distributed between the galactic disk and bulge.

The authors note that if natal kicks are large, isolated black holes may be more widely dispersed, reducing detection efficiency. In that case, the number of objects found would itself constrain models of black hole formation.

The study argues that the success of GRAVITY+ in establishing an era of systematic isolated black hole measurements will depend not only on the instrument’s astrometric precision but also on effective pre-selection from photometric surveys.

With a targeted strategy, the combination of large-scale microlensing searches and high-resolution interferometry could transform isolated black holes from theoretical population to measurable demographic. (PAP)

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