Gaia BH1 has a mass of 9.62 times the mass of the Sun and is orbited by a bright Sun-like star at about the same distance as our planet orbits the Sun. This binary system resides about 1,600 light-years away in the constellation of Ophiuchus.
An artist’s concept of the Gaia BH1 binary system. Image credit: International Gemini Observatory / NOIRLab / NSF / AURA / J. da Silva / Spaceengine / M. Zamani.
The Milky Way is expected to contain at least 100 million stellar-mass black holes, an unknown fraction of which are in binary systems.
The inventory of known and suspected black holes consists of about 20 dynamically confirmed black holes in X-ray binaries, an additional 50 X-ray sources suspected to contain a black hole based on their X-ray properties, a few X-ray quiet binaries in which a black hole is suspected on dynamical grounds, and an isolated black hole candidate discovered via microlensing.
Gaia BH1 is different in several ways from other known black holes in binaries — it is nearby 1,600 light-years away and is very bright.
Together with the lack of contamination from an accretion disk and the fact that the luminous star is cool and slowly rotating, this makes it possible to study the system in greater detail in the optical than is possible for black hole X-ray binaries.
“Take the Solar System, put a black hole where the Sun is, and the Sun where the Earth is, and you get this system,” said Dr. Kareem El-Badry, an astrophysicist at the Harvard & Smithsonian’s Center for Astrophysics and the Max Planck Institute for Astronomy.
“While there have been many claimed detections of systems like this, almost all these discoveries have subsequently been refuted.”
“This is the first unambiguous detection of a Sun-like star in a wide orbit around a stellar-mass black hole in our Galaxy.”
Dr. El-Badry and colleagues originally identified the Gaia BH1 system as potentially hosting a black hole by analyzing data from ESA’s star-mapping Gaia spacecraft.
Gaia captured the minute irregularities in the star’s motion caused by the gravity of an unseen massive object.
To explore the system in more detail, the authors turned to the Gemini Multi-Object Spectrograph instrument on Gemini North, which measured the velocity of the companion star as it orbited the black hole and provided precise measurement of its orbital period.
The Gemini follow-up observations were crucial to constraining the orbital motion and hence masses of the two components in the Gaia BH1 system, allowing the researchers to identify the central body as a black hole roughly 10 times as massive as our Sun.
“When we had the first indications that the system contained a black hole, we only had one week before the two objects were at the closest separation in their orbits,” Dr. El-Badry said.
“Measurements at this point are essential to make accurate mass estimates in a binary system.”
“Gemini’s ability to provide fast-turnaround observations was critical to the project’s success. If we’d missed that narrow window, we would have had to wait another year.”
The team’s work was published in the Monthly Notices of the Royal Astronomical Society.
_____
Kareem El-Badry et al. A Sun-like star orbiting a black hole. MNRAS, published online November 2, 2022; doi: 10.1093/mnras/stac3140
