Black holes might be black, but they are not necessarily invisible. They come in a variety of sizes, from minuscule to supermassive, with a key common feature: a boundary known as the event horizon, beyond which light cannot escape. Black holes near an object such as a star, however, can brighten when they feed, flaring as superheated dust and gas swirls down to oblivion. Those without such a companion are much more difficult to spot, black as they are, but they can still be indirectly detected via their gravitational effects on other nearby objects.
In a paper published in the journal Astronomy & Astrophysics, researchers say they have made just such an observation—unveiling what may be the closest known black hole to Earth. Their investigation of HR 6819, an otherwise inconspicuous star system that is faintly visible to the naked eye in the southern constellation of Telescopium, revealed that one of its two known stars appeared to be orbiting an unseen object once every 40 days. Closer inspection, the team says, shows this unseen object to be a black hole with a mass estimated at 4.2 times that of our sun. A star of comparable mass in HR 6819 would likely be bright enough to easily see, the researchers say. A black hole is therefore the most probable explanation.
“We initially thought [HR 6819] was a binary [system],” says Thomas Rivinius of the European Southern Observatory (ESO), who is the study’s lead author. “But when we looked closer, we saw it was not a binary, it was actually three [objects].
The astronomers used a 2.2-meter telescope at the ESO’s La Silla Observatory in Chile to make the discovery. But this detection was not a recent one: the observations enabling the discovery were actually performed over several months back in 2004. Last year, however, the announcement of a possible black hole in a similar system called LB-1, which caused some debate, prompted Rivinius and his team to reexamine their archival data. “It looked exactly the same,” he says. “I thought, Wait a second. I have something in my drawer of unused data that looks pretty much like [LB-1].”
The team believes the black hole in the HR 6819 system is the result of a star there exploding as a supernova tens of millions of years ago, based on the supposed ages of the system’s two remaining stars. It was not noticed until now because its orbital separation from its companion stars is sufficient to presently prevent it from feeding on them. In contrast, other known black holes in binary systems are the companion of a star that they feast from and are surrounded by glowing disks of material emitting copious x-rays. Astronomers have found only a few dozen of these “x-ray binaries” among the hundreds of billions of stars in our galaxy.
If it indeed hosts a black hole, HR 6819 has some interesting implications. For starters, supernovae are expected to give any nearby stars a gravitational “kick,” potentially disrupting their orbit and sending them flying off into interstellar space. “The fact that this triple system still exists tells us there cannot have been a strong kick, if at all,” Rivinius says. “So that [would be] something new learned about supernovae—that black holes can form without kicks.”
Another implication is that quiescent black holes like this could be much more common than thought, suggesting there are many more to be discovered. It may even be that LB-1 is another example of this heretofore unknown class of black hole systems. Being more distant and fainter, however, it is much harder—though not impossible—to observe. “We have proposed to” study LB-1 as well, Rivinius says.
HR 6819 would also provide some tantalizing hints for how black hole binaries that produce gravitational waves are formed. Such systems, be they two black holes or a black hole and a neutron star, are known to produce these ripples in spacetime when they merge. But how they came to be before merging remains a topic of intense debate in astrophysics. “It’s really unknown,” says Laura Nuttal of the University of Portsmouth in England, who was not involved in the study. “There’s still no clear indication [of] exactly what the formation channel is.”
Kareem El-Badry of the University of California, Berkeley, who was also not a part of the study, finds its claim of discovering the closest ever observed black hole to be “definitely plausible”. He notes, however, that this conclusion relies on a few assumptions, notably that the system’s innermost star orbiting the black hole would be about five solar masses. “I think this is less secure,” he says. If that inner star was not as massive as Rivinius and his team have assumed, the unseen object would be less massive, too—and potentially not a black hole at all. “I don’t think it’s an imprudent thing to say it’s probably a black hole. But there is some uncertainty there,” El-Badry says.
It is also not currently possible to tell whether the supposed black hole is a single object of 4.2 solar masses or two stars of 2.1 solar masses closely orbiting each other, says Edward van den Heuvel of the University of Amsterdam, who was not involved in the study. “It would be a quadruple [star system], but there are lots of quadruple systems among the bright stars in the sky,” he says. “If the thing would start emitting x-rays at some point, we would be sure it was a black hole. But if it never does that, then we stay with the problem: Is it a black hole, or could it be a closed binary of two stars?”
Rivinius, however, says that evidence of such a quadruple system—effectively two binaries coorbiting each other—would be notable in the emitted light from HR 6819. Ultimately, further studies of the system requiring longer stares with more telescopes will be required to answer some of these questions. “As soon as our observatories start operating again, we shall try that,” Rivinius says, noting the shuttering of telescopes across the globe in response to the ongoing coronavirus pandemic. For the time being, at least, our solar system seems to have a new dark companion lurking in its galactic backyard.