Astronomers discover the closest black hole to Earth – in our cosmic backyard

Black holes are the most extreme objects in the Universe. It is believed that supermassive versions of these unimaginably dense objects reside at the centers of all large galaxies. Stellar-mass black holes — which weigh approximately five to 100 times the mass of the Sun — are much more common. In fact, there are an estimated 100 million stellar-mass black holes in the Milky Way alone. However, only a handful have been confirmed to date, and nearly all of these are ‘active’. This means that they shine brightly in X-rays as they consume material from a nearby stellar companion, unlike dormant black holes which do not.

Astronomers have now discovered the closest black hole to Earth, which the researchers have dubbed Gaia BH1. To find it, they used the Gemini North telescope in Hawai‘i, one of the twin telescopes of the International Gemini Observatory, operated by NSF’s NOIRLab.

Gaia BH1 is a dormant black hole that is about 10 times more massive than the Sun and is located about 1600 light-years away in the constellation Ophiuchus. This means it is three times closer to Earth than the previous record holder, an X-ray binary 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 about the same distance as the Earth orbits the Sun.

This animation shows a Sun-like star orbiting Gaia BH1, the closest black hole to Earth, located about 1600 light-years away. Observations from Gemini North, one of the twin telescopes of the Gemini International Observatory, operated by NSF’s NOIRLab, were crucial in limiting 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. Credit: T. Müller (MPIA), PanSTARRS DR1 (KC Chambers et al. 2016), ESA/Gaia/DPAC

“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 this discovery which was published November 2 in Royal Astronomical Society Monthly Notices.

“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 from power. 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?”

Reference: “A Sun-Like Star Orbiting a Black Hole” by Kareem El-Badry, Hans-Walter Rix, Eliot Quataert, Andrew W Howard, Howard Isaacson, Jim Fuller, Keith Hawkins, Katelyn Breivik, Kaze WK Wong , Antonio C Rodriguez , Charlie Conroy, Sahar Shahaf, Tsevi Mazeh, Frédéric Arenou, Kevin B Burdge, Dolev Bashi, Simchon Faigler, Daniel R Weisz, Rhys Seeburger, Silvia Almada Monter and Jennifer Wojno, November 2, 2022, Royal Astronomical Society Monthly Notices.
DOI: 10.1093/mnras/stac3140

The Gemini North sightings were made under a director’s discretionary time schedule (program ID: GN-2022B-DD-202).

The Gemini International Observatory is operated by a partnership of six countries, including the United States through the National Science Foundation, Canada through the National Research Council of Canada, Chile through the National Agency for Research and Development, Brazil through the Ministry of Science and Technology. and Innovations, Argentina through the Ministry of Science, Technology and Innovation, and Korea through the Korea Institute of Astronomy and Space Science. These Participants and the University of Hawaii, which has regular access to Gemini, each maintain a “National Gemini Office” to support their local users.