Gaia BH3: a black hole found by a star’s wobble

Gaia’s wobble

Announced on 16 April 2024, Gaia BH3 had been found while scientists reviewed Gaia observations before a future data release. Gaia tracked a giant star shifting across the sky as an unseen companion tugged at it. ESO described the system as about 2,000 light-years away and, at the time, the Milky Way’s most massive known stellar-origin black hole.[1]

Gaia’s astrometry and spectra showed an 11.6-year orbit and a 32.70 ± 0.82-solar-mass companion at 590 parsecs. Ground-based radial velocities matched the predicted motion within 0.5 kilometres per second. The spectra showed no emission lines, consistent with a dormant black hole.[2]

Mass and limits

One black hole is the simplest interpretation, but the authors could not exclude an inner pair of black holes or a black hole with another compact object. They warn that a small bias in Gaia’s preliminary parallax cannot be quantified, so the mass-function uncertainty may be underestimated. The mass is inferred from motion, not directly imaged.[2]

A metal-poor clue

UVES observations showed that the visible giant has very little iron. Because stars in a pair tend to share chemistry, ESO described this as a clue that the black-hole progenitor was metal-poor. Such stars may lose less mass in stellar winds and leave heavier remnants; the companion is an indirect clue, not a direct measure of the vanished star or proof of one formation path.[1]

This validation discovery is not a population census; Gaia may uncover more quiet binaries.[2]