A black hole that swallowed a star is suprised astronomers with new activity

Artist's illustration of what it looks like when a supermassive black hole "spaghettifies" a star.

In October 2018, a small star was ripped to shreds when it wandered too close to a black hole in a galaxy located 665 million light years away from Earth.

Though it may sound thrilling, the event did not come as a surprise to astronomers who occasionally witness these violent incidents while scanning the night sky.

But nearly three years after the celestial massacre, the same black hole is lighting up the skies again — even though it hasn’t swallowed anything new.

“This caught us completely by surprise — no one has ever seen anything like this before,” says Yvette Cendes, a research associate at the Center for Astrophysics | Harvard & Smithsonian (CfA) and lead author of a new study analyzing the phenomenon.

The team concludes that the black hole is now ejecting material traveling at half of the speed of light, but are unsure why the outflow was delayed by several years. The results, described this week in the Astrophysical Journal, may help scientists better understand black holes’ feeding behavior, which Cendes likens to “burping” after a meal.

The team spotted the unusual outburst while revisiting tidal disruption events (TDEs) — when encroaching stars are spaghettified by black holes — that occurred over the last several years.

Radio data from the Very Large Array (VLA) in New Mexico showed that the black hole had mysteriously reanimated in June 2021. Cendes and the team rushed to examine the event more closely.

“We applied for Director’s Discretionary Time on multiple telescopes, which is when you find something so unexpected, you can’t wait for the normal cycle of telescope proposals to observe it,” Cendes explains. “All the applications were immediately accepted.”

The team collected observations of the TDE, dubbed AT2018hyz, in multiple wavelengths of light using the VLA, the ALMA Observatory in Chile, MeerKAT in South Africa, the Australian Telescope Compact Array in Australia, and the Chandra X-Ray Observatory and the Neil Gehrels Swift Observatory in space.

Radio observations of the TDE proved the most striking.

“We have been studying TDEs with radio telescopes for more than a decade, and we sometimes find they shine in radio waves as they spew out material while the star is first being consumed by the black hole,” says Edo Berger, professor of astronomy at Harvard University and the CfA, and co-author on the new study. “But in AT2018hyz there was radio silence for the first three years, and now it’s dramatically lit up to become one of the most radio luminous TDEs ever observed.”

Sebastian Gomez, a postdoctoral fellow at the Space Telescope Science Institute and co-author on the new paper, says that AT2018hyz was “unremarkable” in 2018 when he first studied it using visible light telescopes, including the 1.2-m telescope at the Fred Lawrence Whipple Observatory in Arizona.

Gomez, who was working on his doctoral dissertation with Berger at the time, used theoretical models to calculate that the star torn apart by the black hole was only one tenth the mass of our Sun. 

“We monitored AT2018hyz in visible light for several months until it faded away, and then set it out of our minds,” Gomez says.

TDEs are well-known for emitting light when they occur. As a star nears a black hole, gravitational forces begin to stretch, or spaghettify, the star. Eventually, the elongated material spirals around the black hole and heats up, creating a flash that astronomers can spot from millions of light years away.

Some spaghettified material occasionally gets flung out back into space. Astronomers liken it to black holes being messy eaters — not everything they try to consume makes it into their mouths.

But the emission, known as an outflow, normally develops quickly after a TDE occurs — not years later. “It’s as if this black hole has started abruptly burping out a bunch of material from the star it ate years ago,” Cendes explains.

In this case, the burps are resounding.

The outflow of material is traveling as fast as 50 percent the speed of light. For comparison, most TDEs have an outflow that travels at 10 percent the speed of light, Cendes says.

“This is the first time that we have witnessed such a long delay between the feeding and the outflow,” Berger says. “The next step is to explore whether this actually happens more regularly and we have simply not been looking at TDEs late enough in their evolution.”

Additional co-authors on the study include Kate Alexander and Aprajita Hajela of Northwestern University; Ryan Chornock, Raffaella Margutti and Daniel Brethauer of the University of California, Berkley; Tanmoy Laskar of Radboud University; Brian Metzger of Columbia University; Michael Bietenholz of York University and Mark Wieringa of the Australia Telescope National Facility.

When a black hole devours a star, it tears apart the gaseous celestial body but it doesn’t consume everything so some stardust shoots into space.

A tidal disruption event (TDE) occurs when a star wanders close enough to a supermassive black hole to be torn apart by tidal forces, leading to the eventual formation of processes by which astronomical objects, such as galaxies, stars, and planets, are formed.

When a star is ripped apart and devoured in this chaotic way, the jet of light usually lasts for a short time.

Until a few years ago, astronomers had spotted only a handful of TDEs. But now, a new generation of wide-field surveys is catching more of them soon after they start—yielding new insights into the violent events and the hidden population of black holes that drives them.

In most cases of black holes swallowing stars, almost all of the outflow is lower in energy but in very rare occurrences, possibly one percent of cases, that outflow is more powerful, like a physical mechanism jetting these extremely rapid emissions.

In October 2018, a star one-tenth the mass of our sun disappeared in a black hole in a galaxy located 665 million light years away. It was a typical TDE—a few months after that event, radio telescopes detected nothing and astrophysicists turned their attention to other parts of the sky.

Three years later, the remnants of that devoured star suddenly lit up again. Astronomers observed the black hole burping up stellar remains of the star years after it was shredded and consumed.

Dr. Yvette Cendes, a postdoctoral fellow in astronomy at the Harvard-Smithsonian Center for Astrophysics in Cambridge, discovered a black hole that has been “burping” out energy from a small star it shredded in 2018, after two years in which it didn’t eject any such material.

In the standard TDE picture, the gravity of the black hole shreds an approaching star into strands like spaghetti. The black hole immediately swallows half the star’s matter while the rest arcs away in long streamers. These rapidly fall back and settle into an accretion disk that steadily feeds material into the black hole, growing so hot that it emits copious x-rays.

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