Astronomers using the Gemini International Observatory, operated by NSF’s NOIRLab, have discovered the closest known black hole to Earth. This is the first unambiguous detection of a dormant stellar-mass black hole in the Milky Way. Its proximity to Earth, just 1600 light-years away, offers an intriguing study target to advance our understanding of the evolution of binary systems.
Black holes are the most extreme objects in the universe. Supermassive versions of these incredibly dense objects likely reside at the center of all major galaxies. Stellar-mass black holes – which weigh around five to 100 times the mass of the Sun – are much more common, with around 100 million in the Milky Way alone. However, only a handful of them have been confirmed so far, and nearly all of them are “active”, meaning they glow brightly in X-rays as they consume material from a nearby star companion, unlike to dormant black holes that do not.
Astronomers using the Gemini North Telescope in Hawai’i, one of the Gemini International Observatory’s twin telescopes, operated by NSF’s NOIRLab, have discovered the closest black hole to Earth, which researchers have dubbed Gaia BH1. This dormant black hole is about 10 times more massive than the Sun and is located about 1600 light-years away in the constellation Ophiuchus, making it three times closer to Earth than the previous record holder, a binary at X-rays in the constellation of Monoceros. . The new discovery was made possible by making exquisite observations of the motion of the black hole’s companion, a Sun-like star that orbits the black hole at roughly 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 get this system,” explained 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. “Although there have been many claimed detections of systems like this, almost all of these discoveries have subsequently been disproved. This is the first unambiguous detection of a Sun-like star in a wide orbit around of a stellar-mass black hole in our Galaxy.
Although there are likely millions of stellar-mass black holes roaming the Milky Way, those that have been detected have been discovered through their energetic interactions with a companion star. When material from a nearby star heads towards the black hole, it becomes superheated and generates powerful X-rays and jets of material. If a black hole is not actively feeding (i.e. dormant), it simply blends into its surroundings.
“I have searched for dormant black holes over the past four years using a wide range of datasets and methods,” El-Badry said. “My previous attempts – and those of others – have revealed a menagerie of binary systems masquerading as black holes, but this is the first time the search has paid off.”
The team initially identified the system as potentially hosting 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 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 an accurate measurement of its orbital period. Gemini’s follow-up observations were crucial in constraining the orbital motion and therefore the masses of the two components of the binary system, allowing the team to identify the central body as a black hole about 10 times more massive than our Sun.
“Our Gemini tracking observations have confirmed beyond reasonable doubt that the binary contains a normal star and at least one dormant black hole,” El-Badry explained. “We were unable to find any plausible astrophysical scenario that could explain the system’s observed orbit that does not involve at least one black hole.”
The team relied not only on Gemini North’s superb observing capabilities, but also on Gemini’s ability to deliver data under tight deadlines, as the team had only a short window to perform. his 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 at the closest distance in their orbits. Measurements at this stage are essential for making accurate mass estimates in a binary system,” El-Badry said. “Gemini’s ability to provide observations over a short period of time was essential 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 struggle to explain how the particular configuration of the Gaia BH1 system could have occurred. Specifically, the progenitor star that later turned into the newly detected black hole would have been at least 20 times more massive than our Sun. This means that he would have lived only a few million years. If the two stars were forming at the same time, this massive star would have quickly transformed into a supergiant, swelling and engulfing the other star before it had time to become a true main-sequence star burning up energy. hydrogen like our Sun.
It is not at all clear how the solar-mass star could have survived this episode, ending up as a seemingly normal star, as observations of the black hole binary indicate. The 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 the formation and evolution of black holes in binary systems, and also suggests the existence of a yet unexplored population of dormant black holes in binaries.
“Interestingly, this system is not easily adapted to standard binary evolution 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 there are.”
“As part of a network of space and ground observatories, Gemini North has not only provided strong evidence for the closest black hole yet, but also for the first system of pristine black holes, unhindered by hot gas. interacting with the black hole,” said Martin Still, NSF Gemini program manager. “While this potentially bodes well for 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 of this binary system so normal?”