The newly-discovered dormant black hole, named Gaia BH2, is orbited by a red giant star, and has a mass of 8.9 solar masses.
Gaia BH2 is located approximately 3,800 light-years away in the constellation of Centaurus.
This black hole, as well as Gaia BH1, was discovered by studying the movement of its companion star.
A strange ‘wobble’ in the movement of the stars on the sky indicated that they are orbiting a very massive object. Other explanations for these massive companions, like double-star systems, were ruled out since they do not seem to emit any light.
The distance of the stars to the black hole, and the orbits of the stars around them, are much longer than for other known binary systems of black holes and stars. Those closer star-black hole pairs, called X-ray binaries, tend to be very bright in X-ray and radio light, and thus easier to find.
But the new discoveries suggest that black holes in wider binaries are more common.
“What sets this new group of black holes apart from the ones we already knew about is their wide separation from their companion stars,” said Dr. Kareem El-Badry, an astronomer at the Harvard-Smithsonian Center for Astrophysics and the Max-Planck Institute for Astronomy.
“These black holes likely have a completely different formation history than X-ray binaries.”
Gaia BH2 and Gaia BH1 were discovered using data from ESA’s Gaia mission.
“The accuracy of Gaia’s data was essential for this discovery,” said Dr. Timo Prusti, ESA’s Gaia project scientist.
“The black holes were found by spotting the tiny wobble of its companion star while orbiting around it. No other instrument is capable of such measurements.”
“Gaia provided accurate measurements of the movement in three directions, but to understand more precisely how the stars moved away and towards us, additional radial velocity measurements were needed.”
“Ground-based observatories provided these for the newly found black holes, and this gave the final clue to conclude that the astronomers had detected black holes.”
Black holes are often not completely invisible. When material falls onto them, they may emit light in radio and X-ray.
For Gaia BH2, NASA’s Chandra X-ray Observatory and the South African MeerKAT radio telescope on the ground looked for this light, but they were not able to spot any signal.
“Even though we detected nothing, this information is incredibly valuable because it tells us a lot about the environment around a black hole,” said Dr. Yvette Cendes, an astronomer at the Harvard-Smithsonian Center for Astrophysics.
“There are a lot of particles coming off the companion star in the form of stellar wind. But because we didn’t see any radio light, that tells us the black hole isn’t a great eater and not many particles are crossing its event horizon.”
“We don’t know why that is, but we want to find out!”
Gaia BH1 and Gaia BH2 have the most widely separated orbits of all known black holes.
The fact that they are also the closest known black holes to Earth suggests that many more similar black holes in wide binaries are still waiting to be discovered.
“This is very exciting because it now implies that these black holes in wide orbits are actually common in space — more common than binaries where the black hole and star are closer. But the trouble is detecting them,” Dr. Yvette said.
“The good news is that Gaia is still taking data, and its next data release (in 2025) will contain many more of these stars with mystery black hole companions in it.”
