When a hapless star ventures too close to one of the supermassive black holes that lurk at the center of galaxies, it’s torn to shreds and stretched like spaghetti. In this so-called tidal disruption event (TDE), the black hole dines on the stellar remnants, which wrap around the black hole’s belly in an accretion disk. During the feast, the black hole can glow brighter than a supernova for months, before returning to a quiet state of hibernation.

Or so the story usually goes.

Continued monitoring by patient astronomers has now revealed a few cases in which black holes wake up and belch matter and energy, sending bursts of radio waves toward Earth months or even years after the initial TDE. “What’s incredibly unusual about [these events] is that the objects came back to life, like a zombie,” says Enrico Ramirez-Ruiz, a theoretical astrophysicist at the University of California, Santa Cruz. “This is really challenging the paradigm.”

Astronomers aren’t sure what’s triggering the delayed outbursts, but they think the emissions could help explain the mysterious mechanisms by which black holes convert infalling stellar material into powerful jets that rocket out from their poles. “It’s telling us something about the physics of the central engine that’s otherwise hidden from us,” says Sasha Tchekhovskoy, a computational astrophysicist at Northwestern University. “These jets can explode entire galaxies, so it’s a really important process in the evolution of galaxies.”

Most of the few dozen known TDEs have been detected from the optical light or x-rays emitted in the initial feast. But, “Radio is now playing a very important role” in understanding TDEs, says astronomer Igor Andreoni of the Joint Space-Science Institute. Black holes generate radio waves by expelling plasma—pumping it out in polar jets or belching out material that crashes into surrounding gas. But these outflows normally take place during a TDE, shortly after the black hole rips apart its meal.

In February 2021, however, Assaf Horesh, an astrophysicist at the Hebrew University of Jerusalem, discovered a radio burst that came 6 months after the initial TDE. Then, on 30 June, Yvette Cendes, an astronomer at the Harvard & Smithsonian Center for Astrophysics, reported finding another delayed flare in a preprint posted to arXiv. Using multiple observatories, she and her colleagues documented a rapid spike in radio activity that launched more than 2 years after the black hole’s initial snack. “It’s a pretty exceptional case,” Cendes says.

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