The closest known black hole to Earth was found by astronomers using the Gemini International Observatory, operated by NSF’s NOIRLab. A dormant stellar-mass black hole has been confirmed to exist in the Milky Way for the first time. As it is only 1,600 light-years away from Earth, it is a fascinating subject for research to improve our understanding of the evolution of binary systems.
The most extreme things in the universe are black holes. All massive galaxies likely have supermassive versions of these incredibly dense objects at their centers. There are an estimated 100 million stellar-mass black holes in the Milky Way alone, which are significantly more widespread and weigh five to a hundred times more than the Sun. Unlike dormant black holes, which do not flash strongly in X-rays because they consume matter from a nearby stellar companion, only a small number have been confirmed so far, and almost all are “active”.
The closest black hole to Earth has been named Gaia BH1 by astronomers using the Gemini North telescope on the island of Hawaii, one of two telescopes at the Gemini International Observatory operated by NSF’s NOIRLab. It is three times closer to Earth than the previous record holder, an X-ray pair in the constellation Monoceros. This dormant black hole is about 10 times the size of the Sun and is located about 1,600 light-years away in the constellation Ophiuchus. Excellent studies of the motion of the black hole’s partner, a Sun-like star that orbits the black hole at about the same distance as the Earth orbits the Sun, made the new discovery possible.
“Take the solar system, put a black hole where the sun is, and the sun where the Earth is, and you get this system,” explained Kareem El-Badry, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian and the Max Planck Institute for Astronomy and lead author of the paper describing the discovery.”While there have been many detections of systems like this, almost all of these discoveries have subsequently been refuted. This is the first unequivocal detection of a Sun-like star in a wide orbit around a stellar-mass black hole in our Galaxy.”
The few stellar-mass black holes that have been found have been revealed by their energizing interactions with a companion star, although there are likely millions of them moving around the Milky Way Galaxy. Superheated material from a nearby star spirals toward the black hole, producing intense X-rays and jets of material. When a black hole is quiescent (i.e. not actively feeding), it simply merges with its surroundings.
“For the past four years, I have been searching for dormant black holes using a wide variety of data sets and methods,” El-Badry said. “My previous attempts – like those of others – have discovered a menagerie of binaries masquerading as black holes, but this is the first time the search has borne fruit.” Data from the European Space Agency’s Gaia probe was initially examined by researchers to determine the potential presence of a black hole in the system. Gaia picked up tiny variations in the star’s speed caused by an unseen giant object.
El-Badry and his team used the Gemini Multi-Object Spectrograph on Gemini North to study the system in more detail. This device accurately determined the orbital period of the companion star by measuring the speed of the companion star as it orbited the black hole. The team was able to identify the central body as a black hole approximately 10 times more massive than our Sun thanks to follow-up Gemini observations, which were crucial in constraining the orbital velocity and subsequently the mass of the two components. binary system.
“Our follow-up observations of Gemini confirmed beyond doubt that the binary contains a normal star and at least one dormant black hole,” El-Badry explained. “We have not been able to find any plausible astrophysical scenario that could explain the observed path of the system that does not include at least one black hole.”Because they only had a small window to make follow-up observations, the team relied not only on Gemini North’s superior observing capabilities, but also on Gemini’s ability to deliver data at short notice.
“When we had our first indication that the system contained a black hole, we were only a week away from the closest separation between the two objects in their orbits. Measurements at this point are essential for accurate mass estimates in the binary system,” he added. El-Badry said. “Gemini’s ability to provide observations on a short timescale was critical to the success of the project. Had we missed this narrow window, we would have had to wait another year.”
The unique configuration of the Gaia BH1 system is difficult to explain using current astronomical concepts of the evolution of binary systems. The progenitor star, which later evolved into the newly discovered black hole, would have had a mass at least 20 times that of the Sun. It would have a short lifespan of a few million years. If both stars were formed at the same time, this huge star would quickly evolve into a supergiant, inflating and swallowing the other star before it had a chance to evolve into a proper main-sequence star like our hydrogen-burning Sun.
It is not at all clear how a solar-mass star could survive this episode and end up as an apparently normal star, as observations of black hole binaries suggest. Theoretical models that allow for survival all predict that a Sun-mass star should have ended up in a much narrower orbit than the one actually observed. This could indicate that there are important gaps in our understanding of how black holes form and evolve in binary systems, and also suggests the existence of a hitherto unexplored population of dormant black holes in binaries. “Interestingly, this system does not fit easily into standard binary evolutionary models,” El-Badry concluded. “It raises a lot of questions about how this binary system was formed, as well as how many of these dormant black holes are out there.”
