Using the Gemini North telescope in Hawaii, one of the twin telescopes of the International Gemini Observatory, run by the National Science Foundation’s NOIRLab, astronomers have identified the black hole closest to Earth found yet. They have named it Gaia BH1.
This dormant black hole is about 10 times as heavy as the sun and is about 1,600 light-years away in the constellation Ophiuchus. It is three times closer to Earth than the previous record holder, an X-ray binary in the constellation Monoceros.
Black holes are the universe’s most extreme objects. Supermassive versions of these inconceivably dense objects are most likely found at the cores of all large galaxies. Stellar-mass black holes, which weigh five to one hundred times the mass of the sun, are much more common, with an estimated 100 million in the Milky Way alone.
Only a few have been confirmed so far, and nearly all of them are “active” — that is, they shine brightly in X-rays as they consume material from a nearby stellar companion, as opposed to dormant black holes, which do not.
Sun-like Star Orbiting Gaia BH1 Black Hole
Credit: International Gemini Observatory/NOIRLab/NSF/AURA/J. da Silva/Spaceengine/M. Zamani
The new discovery was made possible by precise observations of the motion of the black hole’s companion, a star similar to the sun 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,”
said lead author Kareem El-Badry, an astrophysicist at the Center for Astrophysics at Harvard & Smithsonian and the Max Planck Institute for Astronomy.
While many purported detections of systems like this have been made, almost all of these claims have since been debunked. This is the first unambiguous detection of a sun-like star in our galaxy’s wide orbit around a stellar-mass black hole.
The Hunt For Dormant Black Holes
Though millions of stellar-mass black holes are likely to exist in the Milky Way Galaxy, the few that have been discovered were discovered through their energetic interactions with a companion star. As matter from a nearby star spirals toward a black hole, it becomes superheated and emits X-rays and material jets.
If a black hole is not actively consuming matter (i.e., it is inactive), it merely blends in with its surroundings.
“I’ve been searching for dormant black holes for the last four years using a wide range of datasets and methods. My previous attempts—as well as those of others—turned up a menagerie of binary systems that masquerade as black holes, but this is the first time the search has borne fruit,”
said El-Badry.
Gemini Multi-Object Spectrograph
The team first thought the system might have a black hole after looking at data from the European Space Agency’s Gaia spacecraft. Gaia recorded the tiny changes in the star’s motion that were caused by the gravity of a large object that couldn’t be seen.
El-Badry and his team used the Gemini Multi-Object Spectrograph instrument on Gemini North to examine the system in greater detail. This device measured the speed of the companion star as it circled the black hole and gave a very accurate measurement of the time it took to circle the black hole.
The Gemini follow-up observations helped the team figure out that the central body of the binary system is a black hole with about the same mass as our sun. This was done by limiting the orbital motion and, as a result, the masses of the two parts.
Gemini’s follow-up observations proved beyond a reasonable doubt that the binary contains a normal star and at least one dormant black hole.
“We could find no plausible astrophysical scenario that can explain the observed orbit of the system that doesn’t involve at least one black hole,”
said El-Badry.
Small Window Of Opportunity
As they only had a small time span to conduct their follow-up observations, the team relied not only on Gemini North’s excellent observational capabilities but also on Gemini’s capacity to deliver data on short notice.
There was only one week between the first indications that the system contained a black hole and the time when 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. Gemini’s ability to provide fast-turnaround observations was critical to the project’s success. If we’d missed that narrow window, we would have had to wait another year,”
said El-Badry.
Gaia Black Hole Configuration Puzzling
Current models of binary system evolution struggle to explain how the peculiar configuration of the Gaia BH1 system arose. In particular, the progenitor star of the recently discovered black hole would have been at least 20 times as massive as the sun.
This indicates that its lifespan would have been limited to a few million years. If both stars had formed at the same time, this massive star would have quickly grown into a supergiant and swallowed the other star before it could become a hydrogen-burning main-sequence star like the sun.
It is unclear how the solar-mass star could have survived that episode and ended up as an apparently normal star, as evidenced by the Gaia BH1 binary observations. All of the theoretical models that allow for survival say that the star with the mass of the sun should have ended up in a much tighter orbit than what is seen.
This could mean that we don’t fully understand how black holes form and change in binary systems and that there is a population of dormant black holes in binary systems that we didn’t know about before.
It does raise many issues, including how many of these dormant black holes exist in the universe and how this binary system came to be.
“As part of a network of space- and ground-based observatories, Gemini North has not only provided strong evidence for the nearest black hole to date but also the first pristine black hole system, uncluttered by the usual hot gas interacting with the black hole,”
said National Science Foundation Gemini Program Officer Martin Still.
Reference:
Kareem El-Badry, Hans-Walter Rix, Eliot Quataert, Andrew W Howard, Howard Isaacson, Jim Fuller, Keith Hawkins, Katelyn Breivik, Kaze W K 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, Jennifer Wojno, A Sun-like star orbiting a black hole, Monthly Notices of the Royal Astronomical Society, 2022;, stac3140