Astronomers discover the closest black hole to Earth

Black holes are the most extreme objects in the universe. Supermassive versions of these incredibly dense objects are likely to reside at the centers of all major galaxies. Stellar-mass black holes — weighing about five to 100 times the Sun’s mass — are much more common, with an estimated 100 million in the Milky Way alone.

However, only a handful have been confirmed to date, and almost all of these are “active” — meaning they shine brightly in X-rays while consuming material from a nearby stellar companion, as opposed to dormant. black holes which not.

Astronomers using the Gemini North telescope in Hawaii, one of the International Gemini Observatory’s twin telescopes operated by NSF’s NOIRLab, have discovered the closest black hole to Earth, which the researchers have named Gaia BH1. This sleeping 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 Monoceros.

The new discovery was made possible by beautiful 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 systemput a black hole where the sun is, and the sun where the earth is, and you get this system,” explains Kareem El-Badry, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian and the Max Planck Institute for Astronomy, and the lead author of the paper describing this discovery.

“While there have been many purported detections of such systems, almost all of these discoveries have subsequently been disproved. This is the first unequivocal detection of a Sun-like star in broad orbit around a stellar black hole in our galaxy.”

While there are likely millions of stellar black holes roaming the Milky Way galaxy, the few that have been discovered were discovered through their energetic interactions with a companion star. When material from a nearby star whirls towards the black hole, it superheats and generates powerful X-rays and material beams. If a black hole isn’t actively feeding (that is, it’s dormant), it just blends into its environment.

“I’ve been searching for dormant black holes for the past four years using a wide variety of datasets and methods,” says El-Badry. “My previous efforts — as well as others — yielded a menagerie of binary systems masquerading as black holes, but this is the first time the search has paid off.”

The team originally identified that the system may be harboring a black hole by analyzing data from the European Space Agency’s Gaia spacecraft. Gaia captured the tiny irregularities in the star’s motion caused by the gravity of an invisible massive object. To examine 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 an accurate reading. of its orbital period.

Gemini’s follow-up observations were crucial in limiting orbital motion and thus 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 mass of our Sun.

“Our follow-up observations of Gemini confirmed beyond reasonable doubt that the binary star contains a normal star and at least one sleeping black hole,” explained El-Badry. “We could not find a plausible astrophysical scenario that could explain the observed orbit of the system that does not involve at least one black hole.”

The team relied not only on Gemini North’s excellent observation capabilities, but also on Gemini’s ability to provide data within a tight deadline, as the team had only a short period of time to conduct their follow-up observations.

“When we had the first indications that the system contained a black hole, we only had a week before the two objects were closest to each other in their orbits. Measurements at this point are essential to make accurate mass estimates in a binary system.” 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.”

Astronomers’ current models of the evolution of binary systems are difficult to explain how the peculiar configuration of the Gaia BH1 system could have arisen. In particular, the precursor star that later turned into the newly discovered black hole would have been at least 20 times as massive as our sun.

This means that it would have only lived a few million years. If both stars formed at the same time, this massive star would have quickly turned into a supergiant, blowing up and engulfing the other star before it had time to become a true, hydrogen-burning main sequence star like our sun.

It’s not at all clear how the solar-mass star survived that episode and eventually ended up as an apparently normal star, as the observations of the black hole’s binary star indicate. Theoretical models that allow for survival all predict that the solar-mass star should have entered a much tighter 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 a still untapped population of dormant black holes in binary systems.

“Interestingly, this system cannot be easily accommodated in standard binary evolution models,” El-Badry concluded. “It raises a lot of questions about how this binary system was formed, as well as how much of this dormant… black holes there are out there.”

“As part of a network of space and ground observatories, Gemini North has provided strong evidence not only for the closest black hole to date, but also for the first pristine black hole system, uncluttered by the usual hot gas flowing in interacts with the black hole,” said NSF Gemini Program Officer Martin Still.

“While this may portend future discoveries of the predicted dormant population of black holes in our Milky Way, 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 star system so common? ”