Astronomers have delivered a reality check on the hunt for hidden black holes and neutron stars in the Milky Way. A team led by Joshua Simon of the Carnegie Observatories conducted spectroscopic follow-up on 31 binary star systems that Gaia, the European Space Agency’s star-mapping satellite, had flagged as possible homes for dark compact objects. Their conclusion: most of those leads were false positives.
The work, published in the Astrophysical Journal, is a sobering but valuable step forward in learning how to separate real cosmic treasures from statistical noise.
What the Gaia catalog promised
Gaia has been charting the positions, motions, and properties of nearly two billion stars since its launch in 2013. Its third data release, known as DR3, included a catalog of binary star systems, pairs of stars that orbit a shared center of gravity. Among these were systems where one object appeared to be invisible but massive: a black hole or a neutron star.
The idea was straightforward. When a visible star wobbles under the gravitational pull of an unseen companion, astronomers can calculate the companion’s minimum mass. If that mass exceeds about 2.3 times the Sun’s mass, the companion is too heavy to be a white dwarf and too compact to be a regular star. It must be a neutron star or a black hole. Gaia’s astrometric and spectroscopic binary catalogs identified dozens of candidates fitting that description, sparking excitement across the astrophysics community.
Three of those candidates have already been confirmed through ground-based follow-up: Gaia BH1 and Gaia BH2, both stellar-mass black holes, and Gaia NS1, a neutron star. They remain the only gold-standard discoveries so far.
What the new study found
Simon’s team used some of the world’s most powerful telescopes, including the Magellan telescopes in Chile, the Automated Planet Finder at Lick Observatory, and the Keck Observatory in Hawaii, to take high-resolution spectra of 31 candidate systems over several years. By measuring how the stars’ radial velocities (their speed toward or away from Earth) changed over time, they derived complete orbital solutions for 12 of the systems.
The results were striking. For every single candidate with a dark companion mass above 2 solar masses that Gaia had flagged as having an orbital solution, the Gaia orbits turned out to be wrong. The only exceptions were the already-confirmed Gaia BH1, Gaia BH2, and Gaia NS1.
In some cases, the supposed dark companions were actually ordinary low-mass stars or white dwarfs. In one case, the system turned out to be a double-lined spectroscopic binary, two perfectly ordinary stars orbiting each other, not a black hole at all. The false positives likely arose because Gaia’s automated processing flagged borderline cases where the data just happened to look like a high-mass companion.
The team recommends that future searches use more conservative criteria: requiring a signal significance greater than 10 and a goodness-of-fit parameter below 4 when selecting candidates from the Gaia catalogs. These cuts would dramatically reduce the number of false positives.
One promising lead remains
Not all hope is lost. Among the 11 accelerating stars in the sample, systems that showed large accelerations across the sky and along the line of sight but lacked full orbital solutions in the DR3 data, one object stood out. Known as Gaia DR3 3689209059942075008, it has a minimum companion mass of 1.16 times the Sun’s mass, with very small uncertainty. That puts it right at the boundary between the heaviest possible white dwarfs and the lightest neutron stars. It could be either an ultramassive white dwarf or a neutron star, and further observations will be needed to settle the question.
This system suggests that Gaia’s acceleration catalogs, which have received less attention than the main binary catalogs, may still harbor undiscovered compact objects. The acceleration data cover a wider and less strictly filtered set of stars, potentially including wider binaries that the orbital catalogs missed.
Why this matters
Dormant black holes and neutron stars that are not actively pulling matter from a companion are extremely hard to find. They emit no light of their own. The only way to detect them is through their gravitational influence on a nearby star. Gaia is the first instrument capable of systematically scanning the entire sky for these subtle signals.
Every confirmed discovery, including Gaia BH1 at 1,560 light-years away, Gaia BH2, and Gaia NS1, has rewritten what we know about how many black holes and neutron stars lurk in our galactic neighborhood. Estimates suggest the Milky Way may contain 100 million stellar-mass black holes and a billion neutron stars. Finding even a handful of them in wide, non-interacting binaries helps theorists understand how massive stars evolve, how supernovae happen, and how binary systems survive those violent events.
The new paper does not deliver a fresh haul of compact objects. But it provides something almost as valuable: a map of the traps and pitfalls that await anyone who tries to find them. Future searches, including those using Gaia’s upcoming fourth data release (DR4), can now avoid the same dead ends.
“We find no new confirmed black hole or neutron star companions,” the authors write in their conclusion. But they note that the acceleration catalogs “may therefore provide a largely unexplored source of additional wide binaries containing compact objects.”
The search continues.
Images:
- Artist’s impression of the Gaia BH1 system (ESA)
- Gaia’s map of binary stars in the Milky Way (ESA/Gaia/DPAC)
- Illustration of radial velocity method for detecting exoplanets and compact objects (NASA)
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