Radio bursts from ‘zombie’ black holes excite astronomers

Delayed emissions from black holes that fed on stars months earlier could help explain the formation of powerful jets


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.

Horesh’s graduate student Itai Sfaradi may have caught a third example. Reanalyzing a previously spotted TDE, Sfaradi claims in the 10 July issue of The Astrophysical Journal that he found delayed radio emissions in combination with an x-ray flare. These tandem emissions are sometimes seen in so-called x-ray binaries—in which star-size black holes suck gas away from a paired star—hinting that the mechanisms may be related, Horesh says.

Shifts in the black hole’s accretion disk power the flare-ups from x-ray binaries, and Ramirez-Ruiz thinks the same thing may be happening with the supermassive black holes, months after a TDE. In this scenario, a star’s spaghettified gas piles up more slowly over time, allowing the accretion disk to grow colder and thinner. Eventually, the disk weakens enough to open an escape path that allows the black hole’s magnetic field lines to launch material from the disk into space, where it crashes into surrounding gas and produces radio bursts.

Tchekhovskoy agrees—and he’s got models that demonstrate the behavior. He and his colleagues ran computer simulations of accretion disk evolution and found they can reach a Goldilocks state in which jets can form efficiently. The key moment comes when the accretion disk is still dense enough to fuel jets, but not so dense as to reabsorb the generated radio waves. Perhaps that’s why we’re seeing these delayed bursts, he says—“We’re just waiting for the gas to have the right density.”

More clues could come if wide-field radio surveys can capture other zombie awakenings. The Very Large Array, a complex of telescopes in New Mexico, is set to scan the skies for the third time next year, and the Australian Square Kilometre Array Pathfinder will launch a full-sky survey later this year. Both Cendes and Horesh plan to conduct follow-up radio surveys of TDEs using these observatories, among others. In unpublished work, Cendes thinks she’s already found several more candidates.

Discovering a larger population of these TDEs with delayed outbursts would unlock a natural laboratory, enabling the

orists to investigate black hole behavior under a wide range of conditions, Ramirez-Ruiz says. For physicists, he says, “Black hole gastronomy really provides a new playground.”

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