Black holes are the most extreme objects in the universe. Supermassive versions of these unimaginably dense objects are likely to be found at the centers of all large galaxies. Stellar-mass black holes—which weigh about five to 100 times the mass of the sun—are much more common, with about 100 million in our Milky Way alone.
Only a handful have been confirmed to date, however, and almost all of them are “active” — meaning they glow brightly in X-rays as they consume material from a nearby stellar companion, unlike dormant black holes that don’t.
Astronomers using the Gemini North telescope in Hawaii, one of the twin telescopes of the International Gemini Observatory, operated by NSF’s NOIRLab, have discovered the closest black hole to Earth, which the researchers named Gaia BH1. This dormant black hole weighs about 10 times the mass of the sun and is located about 1,600 light-years away in the constellation Ophiuchus, making it three times closer to Earth than the previous record holder, an X-ray binary in the constellation of the Unicorn. .
The new discovery was made possible by making excellent observations of the motion of the black hole’s companion, a sun-like star that orbits the black hole at about the same distance as Earth orbits the sun.
“Take the solar system, put a black hole where the sun is and the sun where the Earth is, and you have 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 this discovery.
“While there have been many claimed detections of systems like this, almost all of these discoveries have subsequently been disproved. This is the first clear detection of a sun-like star in a wide orbit around a stellar black hole in our galaxy.”
Although there are probably millions of stellar-mass black holes orbiting our Galaxy, the few that have been detected have been revealed by their energetic interactions with a companion star. As material from a nearby star spirals toward the black hole, it superheats and creates powerful X-rays and jets of material. If a black hole is not actively fueled (that is, it is dormant), it simply mixes with its surroundings.
“I have been searching for dormant black holes for the past four years using a wide range of data sets and methods,” said El-Badry. “My previous efforts — and others — have brought to light a menagerie of binary systems masquerading as black holes, but this is the first time the search has paid off.”
The team first identified the system as potentially hosting a black hole by analyzing data from the European Space Agency’s Gaia spacecraft. Gaia recorded the small irregularities in the star’s motion caused by the gravity of an unseen massive object. To explore the system in more detail, El-Badry and his team turned to the Gemini Multi-Object Spectrograph instrument on Gemini North, which measured the speed of the companion star as it orbited the black hole and provided a precise measurement of the period its trajectory.
Subsequent Gemini observations were crucial in constraining the orbital motion and hence the masses of the two components in the binary system, allowing the team to identify the central body as a black hole about 10 times the size of our sun.
“Our follow-up observations of Gemini confirmed beyond reasonable doubt that the binary contains a normal star and at least one dormant black hole,” explains El-Badry. “We have been unable to find any plausible astrophysical scenario that can explain the observed orbit of the system that does not include at least one black hole.”
The team relied not only on Gemini North’s excellent observational capabilities but also on Gemini’s ability to provide data on a tight deadline, as the team only had a short window to conduct its follow-up observations.
“When we had the first indications that the system contained a black hole, we had only a week before the two objects were at their closest separation in their orbits. Measurements at this point are essential to make accurate mass estimates in a binary system.” El Badri said. “Gemini’s ability to provide fast-changing observations was critical to the project’s success. If we had missed this narrow window, we would have had to wait another year.”
Astronomers’ current models of the evolution of binary systems struggle to explain how the peculiar configuration of the Gaia BH1 system could have arisen. Specifically, the progenitor star that later turned into the newly identified black hole would have been at least 20 times more massive than our sun.
This means it would have only lived a few million years. If both stars were forming at the same time, this massive star would have quickly turned into a supergiant, inflating and engulfing the other star before it could become a proper hydrogen-burning main-sequence star like our sun.
It is not at all clear how the solar-mass star could survive this episode, ending up as an apparently normal star, as observations of the black hole binary show. Theoretical models that allow for survival all predict that the solar-mass star should have ended up in a much tighter orbit than is actually observed.
This could indicate that there are significant gaps in our understanding of how black holes form and evolve in binary systems, and also suggests the existence of a still unexplored population of dormant black holes in binary systems.
“Interestingly, this system is not easily accommodated by standard binary evolution models,” El-Badry concluded. “It raises a lot of questions about how this binary system formed, as well as how many of these dormant black holes are out there.”
“As part of a network of space and ground-based observatories, Gemini North has not only provided strong evidence for the closest black hole to date, but also the first pristine black hole system, uncomplicated by the usual hot gas interacting with the black hole . said Martin Steele, NSF Gemini program manager.
“While this foreshadows possible future discoveries of the predicted population of quiescent black holes in our Galaxy, the observations also leave a mystery to be solved – despite the shared history with its exotic neighbor, why is the companion star in this binary system so normal? ”
More information: Kareem El-Badry et al, A sun-like star orbiting a black hole Monthly Notices of the Royal Astronomical Society(2022). DOI: 10.1093/mnras/stac3140
Provided by the National Research Laboratory for Optical Infrared Astronomy
Reference: Astronomers discover closest black hole to Earth (2022, November 4) Retrieved November 4, 2022, from
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