Zombie" black holes: strange radio bursts observed in dormant black holes
Astronomers and astrophysicists are currently questioning a behavior observed in several supermassive black holes: they have been caught emitting jets of matter weeks or even months after feeding on a star.
What happens to a star that gets too close to a hungry black hole? It ends up "spaghettied", in other words stretched to the point of forming large shreds of plasma in the shape of the famous Italian pasta. This "spaghettization" has the more scientific name of tidal disruption event (TDE) and is one of the most violent processes in the cosmos.
Due to the powerful gravitational pull of a black hole, it releases a brilliant light before the debris of the disintegrated star disappears beyond the event horizon of the object. This luminous feast may continue for several months before the black hole retreats into a peaceful state of hibernation. But sometimes this sleep state is not so peaceful.
Burps" of black holes far from their digestion
Several observations have revealed that black holes can wake up and belch out 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 these objects came back to life, like zombies," says Enrico Ramirez-Ruiz, an astrophysicist at the University of California, Santa Cruz, as quoted by Science. "This really challenges the paradigm."
Until recently, all of the ejections documented in the few dozen known TDEs, detected from optical light or X-rays emitted by the black hole's jet of matter, had occurred very shortly after the black hole had shredded its meal. But with the development of observations in the radio wave domain, previously unknown behaviors of black holes could be observed. In February 2021, Assaf Horesh, an astrophysicist at the Hebrew University of Jerusalem, discovered a radio burst six months after the initial TDE. On June 30 of the same year, history repeated itself: Yvette Cendes, an astronomer at the Harvard Center for Astrophysics and the Smithsonian Center, reported the discovery of another delayed flare. Using several telescopes, she and her team determined that the rapid spike in radio activity occurred more than two years after the black hole's feeding! The scientists were baffled, to say the least.
A disc in the middle
Now a third example has been added to that list: while reanalyzing a previously spotted TDE, Assaf Horesh's student Itai Sfaradi claims to have recorded delayed radio emissions in combination with an X-ray flare. Such tandem emissions are sometimes seen in so-called X-ray binaries - where star-sized black holes suck in gas from a twinned star - suggesting that the mechanisms may be related. His work is published in the Astrophysical Journal dated July 10, 2022.
One hypothesis is currently being floated by researchers to explain these late flares: as shifts in the black hole accretion disk fuel the flares of X-ray binaries, the same could happen with supermassive black holes months after their meal. The gas of a torn star would slowly accumulate over time, allowing the accretion disk to cool and refine. The latter, "weakened", would then let projections of matter pass into space which, by crashing into the surrounding gas, would produce the famous radio bursts. In short, the accretion disk would still be dense enough to feed the jets, but somewhat too puny to reabsorb the generated radio waves.
To confirm this scenario, large-scale radio studies will be necessary. According to Enrico Ramirez-Ruiz, also interviewed by Science, the discovery of a larger population of these "zombie" TDEs would allow a broader study of black hole behavior under a wide range of conditions. "Black hole gastronomy really offers a new playground," he concludes.