Runaway black hole caught in the act of devouring a star

Astronomers have caught a runaway black hole in the act of tearing apart a star in a place it shouldn’t be: far from the bustling center of its host galaxy. 

The stellar catastrophe – known as a tidal disruption event, or TDE – lit up radio telescopes with two unusually bright, rapid-fire outbursts.

The event challenges assumptions about where supermassive black holes live and how they behave after a “meal.”

“This is truly extraordinary,” said lead author Itai Sfaradi of the University of California, Berkeley.

“Never before have we seen such bright radio emission from a black hole tearing apart a star, away from a galaxy’s center, and evolving this fast. It changes how we think about black holes and their behavior.”

A chaotic black hole population 

The event, cataloged as AT 2024tvd, sits roughly 2,600 light-years from the nucleus of its host galaxy. This is astronomically close by everyday standards, but startlingly off-center for a supermassive black hole.

Most TDEs, and most active supermassive black holes, are found in galactic cores, where dense star fields and deep gravitational wells make close encounters common. Finding one so clearly displaced hints at a more chaotic black hole population. 

Some may wander after galaxy mergers, get kicked by gravitational recoils, or take up residence in massive star clusters beyond the core.

Two distinct radio flares

Tidal disruption events unfold when a star drifts within the tidal radius of a massive black hole. Gravity stretches the star into a stream of gas. 

Some of that debris swings back toward the black hole, forms a hot accretion disk, and, in many cases, powers outflows that plow into surrounding gas and shine in radio waves.

What set AT 2024tvd apart was not just its off-center location, but the tempo. Radio emission typically ramps up over months and lingers. 

Here, the team saw two distinct radio flares rise and fade with unprecedented speed – signals that the black hole didn’t just feed once, but launched at least two separate, powerful outflows months apart.

The black hole reawakened

Detailed modeling suggests a staggered engine: the black hole swallowed the star, settled, then “reawakened” episodically to fling fresh material outward. 

That delayed ejection challenges the simple picture in which outflows follow immediately on the heels of stellar destruction. 

Instead, the accretion flow may clog and clear in bursts, or magnetic fields may take time to wind up and launch jets – phenomena more akin to sputtering engines than steady furnaces.

A global radio stakeout

Capturing the twists of AT 2024tvd took a small army of telescopes. An international team led by Sfaradi and UC Berkeley astrophysicist Raffaella Margutti rallied some of the world’s premier radio facilities.

The team analyzed data from the Atacama Large Millimeter/submillimeter Array (ALMA), the Submillimeter Array (SMA), and the Arcminute Microkelvin Imager Large Array (AMI-LA) in the UK.

Observations with AMI, coordinated by the Hebrew University of Jerusalem, were pivotal for tracking the lightning-fast changes. 

“The fact that it was led by my former student, Itai, makes it even more meaningful,” said Assaf Horesh from the Racah Institute of Physics. “It’s another scientific achievement that places Israel at the forefront of international astrophysics.”

Those multi-band radio snapshots revealed a shock racing through surrounding gas, its spectrum hardening and softening as the outflow expanded and cooled. 

Such a phenomenon is a classic sign of material slamming into the environment at high speed. But the timeline, with two flares months apart, was anything but classic.

A runaway black hole

A supermassive black hole 0.8 kiloparsecs from its galactic core invites a bigger question: how did it get there – and what is it doing?

One possibility is a relic of a past merger. When galaxies collide, their central black holes can form a binary and, after coalescing, recoil from the center under the kick of gravitational waves.

Another possibility is a long-lived massive black hole embedded in a dense satellite or star cluster orbiting the main galaxy.

Either way, AT 2024tvd is a proof of concept that supermassive black holes can remain active away from galactic hubs, where they can still encounter and devour stars.

That matters for sky surveys now coming online. If off-nuclear TDEs are more common than we thought, facilities like the Vera C. Rubin Observatory will find more of them. 

Each one is a laboratory for accretion physics and jet launching under different ambient conditions than the crowded, dusty galactic center.

A new playbook for tidal disruptions

The fast, double-peaked radio light curve of AT 2024tvd points to a more nuanced script for TDEs.

In such events, debris streams take time to circularize, disks switch states, and magnetic fields gradually thread the flow. Outflows then start, stall, and restart.

The study also helps explain why some TDEs are radio-quiet while others, like this one, roar. 

The environment – how much gas surrounds the black hole – and the timing of outflows both shape how bright the shocks become. In sparse regions, even powerful outflows can fade fast. In denser pockets, they blaze.

“This is one of the fascinating discoveries I’ve been part of,” Horesh said. “We’re seeing in real time that black holes can behave in ways we didn’t anticipate.”

Is the black hole really a runaway?

The team’s findings, set to be published in The Astrophysical Journal Letters, will spur follow-up at every wavelength. X-ray and optical monitoring can map how the accretion disk evolves.

Higher-resolution radio imaging can trace the outflow’s shape. And deeper spectroscopy can probe the environment the shock is plowing through. 

If the black hole is truly a wanderer, future observations may catch more off-nuclear fireworks in the same galaxy, or find other displaced giants lighting up nearby stars.

For now, AT 2024tvd stands as a landmark: the first TDE with bright, rapidly evolving radio emission clearly outside a galactic core. 

It’s a reminder that even the heaviest anchors in the cosmos don’t always stay put – and that when they feast, the echoes can come sooner, later, and louder than anyone expected.

A preprint of the study can be found on arXiv.

Image Credit: NSF/AUI/NSF NRAO/P.Vosteen

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