‘Rogue Black Holes’ Might Not Be ‘Rogue’ or ‘Black Holes’


When a star 20 times more massive than our sun dies, it can explode into a supernova and sink back into a dense black hole (with the help of gravity). But this explosion is never perfectly symmetrical, so sometimes the resulting black holes hurtle through space. These wandering objects are often called “rogue black holes” because they float freely, unattached to other celestial bodies.

But that name could be a “misnomer,” according to Jessica Lu, an associate professor of astronomy at the University of California, Berkeley. She prefers the term “free-floating” to describe these black holes. “Rogue,” she says, implies that nomads are rare or unusual – or useless.

This is certainly not the case. Astronomers estimate that there are up to 100 million of these black holes roaming around our galaxy. But because they are solitary, they are extremely difficult to find. Until recently, these so-called rogue black holes were known only from theory and calculations.

“They’re ghosts, so to speak,” says Lu, who has made it his mission to find the Milky Way’s floating black holes.

[Related: We’re still in the dark about a key black hole paradox]

Earlier this year, two teams of space researchers separately revealed detections of what may well be one of these traveling black holes. One of those teams was led by Casey Lam, a graduate student in Lu’s lab. The other was led by Kailash C. Sahu, an astronomer at the Space Telescope Science Institute. Both teams published their papers on a free open-access journal without expert review.

Scientists will get more data from the Hubble Space Telescope in October which Lu said should help “solve the mystery of whether this is a black hole or a neutron star.” “There is still a lot of uncertainty about how stars die and what ghostly remnants they leave behind,” she notes. When stars much more massive than our sun run out of nuclear fuel, they are thought to collapse into black holes or neutron stars. “But we don’t know exactly which ones die and turn into neutron stars or die and turn into black holes,” Lu adds. there a violent supernova explosion? Or does it plummet straight into a black hole and maybe just burp a little?”

With star stuff making up all we know in the world, understanding the beyond of the stars is key to understanding how we came to be ourselves.

How to spot a black hole in the wild

Black holes are inherently invisible. They trap all the light they encounter, so there’s nothing for the human eye to see. So astronomers have to get creative to detect these dense, dark objects.

Typically, they look for anomalies in gas, dust, stars, and other material that could be caused by the extremely strong gravity of a black hole. If a black hole tears material from another celestial body, the resulting disk of debris that surrounds the black hole can be clearly visible. (That’s how astronomers took the first direct image of a in 2019 and an image of the black hole at the center of the Milky Way earlier this year.)

But if a black hole doesn’t inflict chaos with its gravitational pull, there’s almost nothing to detect. This is often the case with these moving black holes. So astronomers like Lu use another technique called astrometric or gravitational microlensing.

“What we do is wait for the random alignment of one of these floating black holes and a star in the background,” Lu explains. “When the two align, the light from the background star is distorted by black hole gravity [in front of it]. It appears as a star brightening [in the astronomical data]. It also gives it a little jaunt in the sky, a little wobble, so to speak.

The background star does not actually move – rather it appears to change course as the black hole or other compact object passes in front of it. This is because the black hole’s gravity warps the fabric of spacetime, according to Albert Einstein’s general theory of relativity, which alters starlight.

The chances that a traveling black hole could cross our celestial neighborhood and disrupt life on Earth are “astronomically low”.

Astronomers use microlenses to study all kinds of temporary phenomena in the universe, from supernovae to exoplanets transiting around their stars. But that’s tricky to do with ground-based telescopes, because Earth’s atmosphere can blur the images.

“In astrometry, you’re trying to measure the position of something very precisely and you need very sharp images,” says Lu. So astronomers rely on telescopes in space, like Hubble, and a few instruments on the ground equipped with sophisticated systems to adapt to atmospheric interference. “There are really only three facilities in the world that can do this astrometric measurement,” Lu says. “We are working at the cutting edge of what our technology can do today.”

The first rogue black hole?

It was this brightening, or a “gravitational lensing event” as Lu calls it, that she and Sahu’s teams spotted in Hubble Space Telescope data in 2011. Something, they assumed, had to pass in front of this star.

Determining what caused a star’s light to oscillate and change in intensity requires two measurements: brightness and position. Astronomers watch this same spot in the sky over time to see how the light changes as the object passes in front of the star. This gives them the data they need to calculate the mass of that object, which in turn determines whether it is a black hole or a neutron star.

“We know the thing that makes the lens is heavy. We know it’s heavier than your typical star. And we know it’s dark,” Lu notes. “But we’re still a bit unsure about the exact weight and how dark it is.” If it’s only a little heavy, say one and a half times the mass of our sun, it could actually be a neutron star. But if it’s three to ten times more massive than our sun, then it would be a black hole, Lu says.

As the two teams collected data from 2011 to 2017, their analyzes revealed markedly different masses for this compact object. Sahu’s team determined that the traveling object has a mass seven times that of our sun, which would put it squarely in black hole territory. But Lam and Lu’s team calculated it to be less massive, somewhere between 1.6 and 4.4 solar masses, which covers both possibilities.

[Related: Black holes can gobble up neutron stars whole]

Astronomers can’t be sure of the correct calculation until they have the chance to find out how bright the background star is normally and its position in the sky when something isn’t passing in front of it. . They weren’t focused on this star before noticing its unusual brightness and wobble, so now they just have the chance to make these basic observations because the lensing has faded, Lu says. These observations will come from new Hubble data in the fall.

What they do know is that the object in question is in the Carina-Sagittarius spiral arm of the Milky Way galaxy and is currently about 5,000 light years from Earth. This detection also suggests that the nearest traveling black hole could be less than 100 light-years away, says Lu. But that’s no reason to worry.

“Black holes are a drain. If you get close enough, they will consume you,” Lu points out. “But you have to be very close, much closer than I think we usually imagine.” The boundary around a black hole marking the line where light can still escape its gravity, called the event horizon, usually has a radius of less than 20 miles.

The odds that a traveling black hole could cross our celestial neighborhood and disrupt life on Earth are “astronomically low,” says Lu. “It’s the size of a city. Thus, a black hole could pass by the solar system and we would hardly notice it.

But she doesn’t rule it out. “I’m a scientist,” she says. “I can’t say no chance.”

Whether the first teams detected a traveling black hole or a neutron star, says Lu, “the real breakthrough that these two papers show is that we can now find these black holes using a combination of brightness and position measurements.” This opens the door to the discovery of more light-capturing nomads, especially as new telescopes come online, including the Vera C. Rubin Observatory currently under construction in Chile and the Nancy Grace Roman Space Telescope whose launch is expected later this decade.

According to Lu, “the next chapter of black hole studies in our galaxy has already begun.”


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