2 newfound black holes are the closest ever to Earth

Astronomers have discovered two new black holes that are the closest to Earth known, and also represent something astronomers have never seen before.

The black holes, named Gaia BH1 and Gaia BH2, were discovered in data collected by the European Space Agency’s (ESA) Gaia spacecraft.

Gaia BH1 is located just 1,560 light-years away from Earth in the constellation Ophiuchus, while Gaia BH2 is located 3,800 light-years away in the constellation Centaurus. In cosmic terms, both black holes are located in Earth’s backyard.

Related: The Black Holes of the Universe (Images)

However, it is not just the proximity of these black holes to Earth that makes them extraordinary. They orbit the star at much greater distances than previously observed in other black hole companion star pairs.

“What distinguishes this new group of black holes from those we already know about is their wide separation from their companion stars,” discovery team leader Kareem El-Badry of the Harvard-Smithsonian Center for Astrophysics in Massachusetts and Max-Planck. The Institute for Astronomy in Germany said in a statement (opens in new tab).

“Normal” black hole companion star systems are called X-ray binaries and are usually bright in high-energy X-ray and radio emissions. This makes them easier to find than black holes that aren’t gobbling up matter and thus aren’t emitting powerful bursts of energy. Gaia BH1 and Gaia BH2 are completely dark and were discovered through the gravitational effect they have on their companion stars.

“These black holes likely have a completely different formation history than X-ray binaries,” El-Badry explained. “We suspected there might be black holes in more massive systems, but we weren’t sure how they would have formed. Their discovery means we have to adjust our theories about the evolution of binary star systems , as it is not yet clear how these systems form.”

Gaia is ideal for spotting ‘invisible’ black holes

Gaia is equipped to make such discoveries because it can precisely measure the position and motion of billions of stars against the background sky. Tracking this stellar motion so precisely hints at the gravitational influences exerted on these stars by other stars, orbiting planets and black holes, the researchers said.

“The accuracy of the Gaia data was crucial to this discovery,” said ESA Gaia project scientist Timo Prusti. “Black holes were found by detecting the small wobble of its companion star during orbit around it. No other instrument is capable of such measurements.”

Gaia’s observations were supported by measurements of the motion of each companion star made by other observatories. For example, follow-up investigations of Gaia BH2 with NASA’s Chandra X-ray Observatory and South Africa’s MeerKAT radio telescope on the ground did not detect any detectable light coming from this black hole.

“Even though we didn’t detect anything, this information is incredibly valuable because it tells us a lot about the environment around a black hole,” said discovery team member Yvette Cendes of the Harvard-Smithsonian Center for Astrophysics.

“There’s a lot of particles coming out of the companion star in the form of the stellar wind,” Cendes said. “But because we didn’t see any radio light, it tells us that the black hole is not a big eater and not many particles are crossing its event horizon. We don’t know why that is, but we want to find out! “

The team will now try to detect more widely separated black hole binaries in the next data dump from Gaia, to be released in 2025. This new data will be based on 66 months of observations from the spacecraft and will contain more detailed information on the motion of the stars.

“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 the star are closest together. But the problem is detecting them,” concluded Cendes. “The good news is that Gaia is still receiving data, and its next data release will contain many more of these stars with mysterious black hole companions.”

The discovery of these two black holes was detailed in a paper published late last month in the Monthly Notices of the Royal Astronomical Society (opens in new tab).

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