Astronomers have discovered the closest black hole to Earth
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 roughly five to 100 times the mass of the Sun—are much more common, with an estimated 100 million of them in the Milky Way alone.
However, only a few have been confirmed to date, and almost all of them are “active” – meaning they glow brightly in X-rays as they consume material from a nearby stellar companion, as opposed to a dormant one. black holes which does not.
Astronomers using the Gemini North Telescope in Hawaii, one of the Gemini International Observatory’s twin telescopes 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 as much the mass of the sun and is 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 system in the constellation Monochrome.
The new discovery was made possible by extraordinary 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 Solar systemput 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 this discovery.
“Although there have been many claims to discover a system 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.”
Although there are millions of stellar-mass black holes roaming the Milky Way galaxy, the few that have been detected have been detected by their energetic interactions with companion of the stars. As material from a nearby star spirals toward the black hole, it becomes superheated and creates powerful X-rays and jets of material. If a black hole is not actively feeding (i.e. quiescent), it simply merges with its surroundings.
“I have been searching for dormant black holes for the past four years using a wide variety of data sets and methods,” said El-Badry. “My previous attempts – and those of others – have revealed a menagerie of binaries masquerading as black holes, but this is the first time the search has borne fruit.”
The team initially identified the system as a potential site for a black hole by analyzing data from the European Space Agency’s Gaia spacecraft. Gaia recorded tiny irregularities in the star’s motion caused by the gravity of an invisible massive object. To investigate 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 allowed a precise measurement of its orbital period.
The Gemini observations were key to constraining the orbital motion and thus the mass of the two components in the binary system, allowing the team to identify the central body as a black hole roughly 10 times more massive than our Sun.
“Our observation of Gemini confirmed beyond a reasonable doubt that the binary system contains a normal star and at least one dormant black hole,” elaborated El-Badry. “We could not find a 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 outstanding observational capabilities, but also on Gemini’s ability to provide data at short notice, as the team only had a short period in which to make their follow-up observations.
“When we had the first indication 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 necessary to make accurate estimates of the mass in the binary system,” he said. El-Badry. “Gemini’s ability to provide rapid observations was critical to the success of the project. If we had missed that narrow window, we would have had to wait another year.”
Astronomical current models of the evolution of binary systems are hard pressed to explain how the unusual configuration of the Gaia BH1 system could have arisen. Specifically, the progenitor star that later turned into the newly discovered black hole would have been at least 20 times more massive than our Sun.
This means that he would only live a few million years. If both stars had formed at the same time, this massive star would have quickly turned into a supergiant, inflating and engulfing the other star before becoming a true hydrogen-burning main-sequence star like our Sun.
It is not at all clear how a solar-mass star could have survived that episode, ending up as an apparently normal star, as observations of the binary black hole show. All theoretical models that allow for survival predict that a solar-mass star should have ended up in a much narrower orbit than what is 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 an as yet unexplored population of dormant black holes in binary systems.
“Interestingly, this system is not easily accommodated by standard models of binary evolution,” El-Badry concluded. “It raises many questions about how this binary system was formed, as well as how many of these are dormant black holes they exist out there.”
“As part of a network of space-based and ground-based observatories, Gemini North not only provided strong evidence for the closest black hole to date, but also the first intact black hole system, unencumbered by the usual hot gas that interacts with a black hole. ” said NSF Gemini Program Officer Martin Still.
“While this potentially hints at future discoveries of a predicted dormant population of black holes 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?”
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 Laboratory for Optical-Infrared Astronomy
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