Could Mysterious Black Hole Burps Rewrite Physics?

Scientific American

17-06-2025

Black holes are invisible, yet they are among the brightest things in the universe. If a star wanders too close to a black hole, it gets torn apart in a fireworks show called a tidal disruption event. As the star approaches, it gets twisted and pulled, and about half of it ends up flung outward. The other half forms a Frisbee-shaped accretion disk around the black hole itself. This newly formed disk is not stable: material sloshes around and smashes into itself, creating a light show detectable in radio wavelengths.
These are rare occurrences—scientists estimate that the giant black hole at the center of our Milky Way galaxy gobbles a star about every million years or so. But when it happens, it releases a tremendous amount of light and energy visible millions or even billions of light-years away.
Until recently, astronomers had thought that after the initial feast, the swallowed star was never to be seen again. Observations in the past five years, however, suggest otherwise. In a surprising turn unpredicted by theory, it appears that black holes can suffer from indigestion, spewing out material years after the initial stars were shredded. In fact, scientists are now finding that up to half of black holes that devour stars start shining again in radio light years after they had gone quiet—the equivalent of a cosmic burp. We know this material isn't coming back from beyond the event horizon—that's impossible. It's most likely sloshing about in an accretion disk outside that boundary. But explaining how these black hole burps can occur so late is challenging. What's going on? Solving the mystery of these regurgitations may reveal new secrets about the physics of the most extreme environments in the universe.
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Most galaxies around the size of the Milky Way or larger have a supermassive black hole skulking in their center. Each of these black holes can be millions or even billions of times more massive than our sun, and their event horizons—the points of no return—can extend past the radius of Pluto's orbit around the sun. Despite this gargantuan size, however, a black hole doesn't suck in material like a vacuum cleaner any more than our sun sucks in the planets. If our sun were instantly replaced by a black hole, for example, Earth would continue on the same orbit as always. Instead what makes a black hole so unique is its density. Within the event horizon distance, its gravitational pull is so strong nothing can escape.
And for supermassive black holes, their mass alone means they have an extremely strong gravitational pull. This is the case for our own Milky Way's black hole, called Sagittarius A* (or Sgr A* for short). It's located about 27,000 light-years from Earth and is about four million times as massive as the sun. Astronomers have carefully tracked several dozen individual stars for decades that are in stable orbits around Sgr A*. But astronomers believe there are thousands of objects orbiting Sgr A* that we can't see—many of them the leftover remains of dead stars, such as neutron stars or white dwarfs, that are too dim to detect. If one of those unknown objects passes near a star, it may disrupt its orbit, sending it on a collision path toward Sgr A*.
It appears that black holes can suffer from indigestion, spewing out material years after the initial stars were shredded.
Well before it reaches the event horizon, the doomed star will start to experience tidal forces. Gravity gets stronger the closer you get to a massive object, so the side of the star closer to the black hole will feel stronger gravitational forces than its far side does. The star will begin to stretch, and eventually, at a boundary called the tidal radius, the difference in pull between the two sides of the star will be greater than the gravitational force holding the star together. The star will unravel along its direction of motion in a process called spaghettification—first changing from a sphere into an oval and then becoming a long string of material that resembles a thin strand of pasta. As the star's density decreases, its internal fusion stops, and a star that might have burned for billions of years unravels in just a few hours. Half the material is immediately flung outward, never to return, and the rest forms a new accretion disk sloshing around the black hole. When this happens, the rapid change of mass into an accretion disk creates a very bright flare, usually at optical wavelengths.
The first tidal disruption event candidates were discovered in the 1990s, and astronomers have now seen about 100 of them. The unbinding of a star gives off a flare visible from millions of light-years away, similar at first glance to an exploding star. There are a few key differences, however: First, a tidal disruption event occurs in the center of a galaxy, where supermassive black holes lurk, whereas supernovae can occur anywhere. Second, the light from a black hole flare will reveal a spectrum unlike that of a dying star. Astronomers can spot the light signature of an abundance of hydrogen because the star will likely have copious leftover fuel that never got a chance to be used, implying the star didn't meet a natural death.
We discover about a dozen new tidal disruption events a year. These eruptions occur around black holes that otherwise aren't eating much. That makes them different from black holes we call active galactic nuclei, which are engaged in many-years-long eating fests, sucking in large amounts of gas over long timescales and continuously emitting light as they do so. Those feeding frenzies are intensely chaotic and play out at a haphazard pace. In comparison, tidal disruption events are relatively controlled events that allow us to watch what happens when a small bit of very dense material is injected into the black hole all at once.
When someone spots a new one, radio astronomers like me swing our telescopes to look for emission from the mass and energy flowing outward from the newly formed accretion disk, looking for any radio emission present where there was none before, called the outflow. Radio waves come from electrons spiraling in magnetic fields created in those outflows, giving us a physical picture impossible to get at other wavelengths. We can detect the speed of the escaping material, the energy of the blast, the strength of the magnetic fields, and even the density of gas and dust the outflow is plowing through. Furthermore, once the outflows leave the newly formed accretion disk, they can travel several light-years in distance before they fade. Observing these outflows gives astronomers a unique way to probe the environment around a previously dormant supermassive black hole on a detailed level not possible with other methods.
About 99 percent of all the mass released in a tidal disruption event is called nonrelativistic—it moves along at 10 percent the speed of light or less. The remaining 1 percent, however, is very different. In these cases, material from a shredded star gets funneled into a jet launched at nearly the speed of light. This is so fast that the laws of relativity must be considered when we study it, and thus we call it a 'relativistic' outflow. The first known relativistic tidal disruption event, called Swift J1644+57, was detected in 2011 when NASA's Neil Gehrels Swift Observatory spotted a strange burst of radiation from the center of a galaxy 3.8 billion light-years away. After a year and a half of steady emission, the jet in Swift J1644+57 turned off abruptly, presumably when the material from the star that was feeding the jet had been mostly consumed, and the accretion rate—the amount of mass being eaten by the black hole in a given time—declined below some critical value. Before this discovery, no one expected these black hole feeding events to be capable of launching relativistic jets, let alone one that turned on and off on such a short timescale. Exactly how and why they're created is not fully understood.
Astronomers also assumed that the light pattern from all tidal disruption events matched that one—a flare for a few months followed by nothing. After they go dark, we usually stop looking. After all, radio telescope time is a precious resource. Why waste valuable time looking at an explosion years after it occurred? It was a reasonable assumption to make, but it turns out it was the wrong one. It did, however, set me up to make the discovery of a lifetime.
I first decided to be an astronomer when I was 13 years old and read a book about space. I have always loved stories, and the story of the universe is the biggest and grandest one we have. I decided to be a radio astronomer in high school, thanks to Carl Sagan's 1985 novel Contact, in which the heroine, Ellie Arroway, uses the Very Large Array (VLA) in New Mexico to discover an extraterrestrial message. Once I started working in the field, I never stopped, because radio astronomy feels like magic: it lets us tease out the faintest signals by linking together antennas the size of buildings, which sing a story impossible to hear otherwise. My career as a radio astronomer has been filled with adventures, but none has matched the discovery of AT2018hyz, my first burping black hole.
It all began on a bright autumn day in 2021 in Cambridge, Mass. I was a postdoctoral researcher at the Center for Astrophysics | Harvard & Smithsonian, working on data from the VLA that no one else had time to look at. A few months before, another team had detected a tidal disruption event in radio light called ASASSN-15oi, more than 100 days after it was first seen in optical light, despite no radio detection at earlier times. Most people assumed the flare was the result of some unusual circumstance intrinsic to this object or its environment, but I thought it wouldn't hurt to do a survey with the VLA and see whether any other black holes displayed repeated flares.
The VLA collects radio light from 27 antennas, and then these data must be combined to create a radio picture. If we see a source of radio light, it appears as a cluster of pixels in a sea of black. If there's nothing out there, we see only a noise pattern. On this fateful day, I opened an image of a tidal disruption event called AT2018hyz that had been discovered in optical light in 2018. As I looked at the screen, I paused in confusion for a moment before going to manually confirm that the coordinates were correct. Where I'd expected noise, which is all anyone had seen in radio light from this region of space before, there was an unmistakably bright source—this despite being some 665 million light-years from Earth. It had, very definitely and without any fuss, turned 'on.'
I reached out to my collaborators, who were all as excited as I was, and I found a radio survey image that just happened to be taken of the same patch of sky only nine months earlier. There was nothing but noise, implying the radio emission from AT2018hyz had risen rapidly in just a few months. No one had ever seen anything like it before in astronomical history.
Around the time that the first observations were coming in, I went home and told my husband about the discovery. 'The problem is AT2018hyz doesn't really roll off the tongue,' I told him, 'and it's pretty obvious we'll be talking about this for a while. Would you like to name it?' My husband paused, taking the correct tone of gravitas and sober dignity one should have when your wife offers you naming rights to a black hole. 'Jetty McJetface,' he said firmly. It's not official, but from then on AT2018hyz was called 'Jetty' at our house.
In some sense, the most remarkable thing about Jetty was that it turned out it wasn't alone. By the time I had analyzed the data from the full observation campaign, I had several new radio detections of years-old tidal disruption events, all of which had been initially discovered, then turned off, and were now shining again. It seemed that black holes, after consuming stars, suffer a fit of indigestion after a few years and 'burp.' This was surprising for several reasons. Lighting up again after a few years is an unusual timescale for such a thing to happen on. You don't return to the site of a bomb explosion years after it occurred expecting to see new debris released. And we don't think the black hole simply started snacking on a new star—if that were the case, we'd also see optical light, but we don't.
Ultimately, my team and I surveyed about two dozen black holes, all of which were first detected and confirmed in optical light. From these discoveries, we knew exactly when the initial brightening event had occurred. All of them had been surveyed in radio light in the intervening years and were dark. Of these, we discovered 10 burping black holes that were alight again in radio waves. Whatever is happening, it's common and opens our eyes to a new phenomenon that we can use to test the physics of black holes.
We live in a universe filled with cosmic destruction on grand scales and at distances often hard to comprehend.
We still have many open questions, but here's what we know so far. First, the assumption that tidal disruption events release light and energy primarily in the first few months is wrong. Although we always observe optical light at the initial disruption, our data suggest that radio emission is most common at least 1,000 days after that. Some black holes even seem to release a second flood of radio waves—one relatively promptly and another hundreds of days after the first one has faded. There appears to be no significant correlation between when the black hole starts to shine in radio light and when it emits in other wavelengths—the radio emission isn't accompanied by an optical flare indicating a second star has been disrupted or by x-ray light indicating a significant change in how much mass the black hole was accreting.
Finally, the radio data collected so far tell us that these delayed burps look like relatively normal nonrelativistic tidal disruption event outflows—just seen much later than we'd expect. The density of gas we measure in their environments is also similar to that in our own Milky Way. In other words, there's nothing special about the black holes' surroundings.
Now, of course, the million-dollar question is why black holes burp. It appears as if they gobble up mass, pause, and then start spitting a bit out. To be clear, we are not seeing material escaping from beyond the event horizon of the black hole: this would be physically impossible, and we have absolutely no indication that this is what's happening. Instead we think something is going on in the accretion disk or beyond. Perhaps, astrophysicists have suggested, the accretion disk forms much later than we'd previously assumed, or possibly the black holes are creating unusual density fluctuations in their environments. The flares could be caused by interacting dust clouds, or maybe a cocoon of material around the black hole delays the flow of radio emission until later. It is currently unclear which theory, or theories, is correct.
The exception to all of this, though, is Jetty (or AT2018hyz). Although other black hole burps show some similarities to one another, Jetty literally outshines them all. Its brightness has continued to rise since I first discovered it, and it's now about 40 times brighter than it was at that detection. We still aren't sure what's driving it, but there are two possibilities. The first is that Jetty 'burped' about two years after eating a star, releasing an outflow traveling at roughly one-third the speed of light. That would be the first 'mildly relativistic' outflow we know of, somewhere in the middle of the nonrelativistic and the nearly light speed.
The second option is potentially more incredible. Perhaps when the original tidal disruption event happened in October 2018, a relativistic jet of material was launched at an almost 90-degree angle to Earth. This jet would be one of the highest-energy ones we've seen. To start, its direction would make it invisible to us, but over time the jet would widen and enter our line of sight. This could be what we are seeing now, years later. Just how energetic and how bright it will get is impossible to know until we see it happen.
To distinguish between the two possibilities, my collaborators and I are studying Jetty with another method, called Very Long Baseline Interferometry (VLBI). With VLBI, we are linking together radio telescopes spread across North America and Europe to create a virtual radio telescope that's effectively the size of the distance between Germany and Hawaii. We believe this combined scope will have enough resolution to see the material flying out of the black hole directly, despite our being hundreds of millions of light-years away. The first observations are in, but analysis of data over such large distances is tricky—we hope to have the answer soon.
We also hope to grow our collection of known tidal disruption events to monitor for burps. The Vera C. Rubin Observatory, turning on this year, is an 8.4-meter-diameter telescope housed in Chile that will survey the entire night sky every night. Once fully operational, Rubin is expected to find millions of new objects, ranging from supernovae to asteroids, and should uncover around 1,000 new snacking black holes a year. Additionally, the Nancy Grace Roman Space Telescope will launch in 2027. This scope should produce images of similar sharpness to the Hubble Space Telescope but with a field of view 100 times wider. We expect it to find hundreds more tidal disruption events a year. For scientists who were used to discovering a comparative trickle of new objects, this fire hose of new data should be exciting and challenging.
We live in a universe filled with cosmic destruction on grand scales and at distances often hard to comprehend. But black holes will continue to feast—and burp—and my colleagues and I will be watching.