Closest black hole is in our cosmic backyard
Stellar-mass black holes are bizarre and fascinating. They’re also mostly very far away, presumably spread throughout our galaxy and other galaxies. On November 2, 2022, astronomers using the Gemini North telescope in Hawaii said they’ve discovered the closest-yet black hole to Earth, dubbed Gaia BH1. The black hole is about 1,600 light-years away, in the direction of our constellation Ophiuchus the Serpent Bearer. That’s still a long way off. But it’s the closest black hole so far. And, in galactic terms, it’s in the cosmic backyard.
Meet Gaia BH1: Closest black hole discovered so far
Previously, data from the European Space Agency’s Gaia spacecraft led researchers to think there might be a black hole in this stellar system (the black hole’s companion star). Now, Gemini North, operated by NOIRLab, has confirmed it. The black hole’s companion is a sun-like star that orbits the black hole at about the same distance that Earth is from our sun.
Take the solar system, put a black hole where the sun is, and the sun where the Earth is, and you get this system. 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.
The gravity of the black hole causes minute irregularities in the star’s motion, which Gaia previously detected. With this in mind, Gemini North measured the velocity of the companion star as it orbited the black hole. By doing so, it provided a precise measurement of its orbital period. Gemini North also had a narrow window of opportunity for follow-up observations, as El-Badry explained:
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. Measurements at this point are essential to make accurate mass estimates in a binary system. Gemini’s ability to provide observations on a short timescale was critical to the project’s success. If we’d missed that narrow window, we would have had to wait another year.
A dormant stellar-mass black hole
Gaia BH1 is approximately 10 times more massive than our sun, making it a stellar-mass black hole. Other black holes, such as the ones in the centers of galaxies, including our own, can be much more massive than that. This newly discovered black hole is also dormant. That means it is no longer emitting X-rays, as active black holes do.
Scientists say there are likely millions of stellar-mass black holes in our galaxy. Finding them, however, is difficult, and only a few have been confirmed. It is easiest to find them when they have a companion star, as Gaia BH1 does. The energetic interactions of the black hole with the star make it easier to detect. This happens when material from the star spirals in toward the black hole. As the black hole “feeds” on this material, it generates X-rays, and jets of material erupt from close to the black hole. As El-Badry noted:
I’ve been searching for dormant black holes for the last four years using a wide range of datasets and methods. My previous attempts – as well as those of others – turned up a menagerie of binary systems that masquerade as black holes, but this is the first time the search has borne fruit.
Our Gemini follow-up observations confirmed beyond reasonable doubt that the binary contains a normal star and at least one dormant black hole. We could find no plausible astrophysical scenario that can explain the observed orbit of the system that doesn’t involve at least one black hole.
The researchers found no alternative explanations for the observations, as the paper said:
We find no plausible astrophysical scenario that can explain the orbit and does not involve a black hole.
A peculiar system
The discovery itself is exciting, of course, but astronomers still have many questions. Scientists say that the original star – that later became the black hole – would have been 20 times more massive than our sun. If so, it should have lived for only a few million years. That’s a very short life span for a star. But if that star and its companion star formed at the same time, then the original star should have turned into a supergiant star. It then would have consumed the companion star in the process. However, that obviously didn’t happen, since the companion star is still there. Could the companion star have survived somehow?
The fact that the companion star still exists presents a puzzle. It means that there are probably gaps in our theoretical models of black holes. As El-Badry commented:
It is interesting that this system is not easily accommodated by standard binary evolution models. It poses many questions about how this binary system was formed, as well as how many of these dormant black holes are out there.
NSF Gemini Program Officer Martin Still added:
While this potentially augurs future discoveries of the predicted dormant black hole population in our galaxy, the observations also leave a mystery to be solved: Despite a shared history with its exotic neighbor, why is the companion star in this binary system so normal?
Bottom line: Astronomers using the Gemini North telescope in Hawaii have discovered the closest black hole yet to Earth. It is 1,600 light-years away in the constellation Ophiuchus.