This past Valentine’s Day, the Earth was struck by an incredible, rare flash from two-thirds of the way across the universe – a stunning eight-and-a-half-billion light years – from a galaxy that’s still too distant to be seen.
It wasn’t caused by anything nice; it was the result of a supermassive black hole millions of times more massive than the Sun devouring a passing star. According to a paper published by an international team today in Nature and Nature Astronomy, telescopes from around the world came together to catch the most distant instance of an extremely rare jetted tidal disruption event, where the black hole shoots out some of the star it’s tearing apart as a jet of plasma at nearly the speed of light — in this case, straight towards Earth.
What’s new — Tidal disruption events (TDEs) are rare enough. Fewer than a hundred, even counting unconfirmed candidates, have been detected since the first was found in 1990. These events occur when a star wanders too close to a supermassive black hole. When that happens, explains Nial Tanvir, an astronomer at the University of Leicester, they are “shredded totally, ripped apart, and half of the star, roughly, will be eaten by the black hole.” In the vast majority of cases, the star’s matter becomes a part of the black hole’s accretion disk, illuminating it in the process.
But in a handful of cases, “as it collapses into the black hole, it can give rise to these relativistic jets,” Tanvir tells Inverse. It’s been more than a decade since the last time astronomers were able to observe a tidal disruption event that also expelled matter out in an powerful jet of plasma. The first one was observed in 2011, then another a few months later, but, Tanvir notes, “there’s been a hiatus of a decade and we haven’t found any more, or at least not convincing ones.”
It’s such an unusual phenomenon that AT2022cmc’s observers didn’t consider it to be a possibility at first. Igor Andreoni, an astronomer at the University of Maryland and Goddard Space Flight Center who wrote the software that picked the event out of the Zwicky Transient Facility, tells Inverse, “It took us quite some time to track back to what we think it is now, because it doesn’t look at all like normal tidal disruption events … it was something that doesn’t quite fit any of the other classes of things we have for transit events.”
Digging into the details — On top of its rarity, none of the previous jetted tidal disruption events had been found through observation in the visual spectrum, and a telescope like ZTF catalogs (if not acts on) millions of candidate events a night. Giorgos Leloudas, an astronomer at the National Space Institute at the Technical University of Denmark, notes “all previous events that belong to this family have been discovered in gamma rays.”
At first, astronomers expected it would be a gamma ray burst — one that was fading rapidly. Says Tanvir, “In the early hours, all we knew was that there was a quickly fading source in this direction.” As the call went out on Valentine’s Day for an orchestra of X-ray telescopes, radio telescopes, and spectroscopes that would give a better sense of what it might be, he notes there was “a slight ethical dilemma, initially — our program’s awarded time to follow up on gamma ray bursts, so you have to ask yourself at what point can you feel legitimate in giving it a go?”
Over the next few days, astronomers tried to winnow down what exactly the event could be. “You have to have all the pieces of the puzzle,” Andreoni says, “but it takes some time to put the puzzle together. We were comparing all the information we had with analogs in different classes of sources and none was fitting.” When a jet was finally suggested, “we saw that the X-rays fit perfectly, and all the incredible luminosity that we measured could be explained.”
Why it matters — The event was bright enough — and distant enough — that astronomers are still searching for exactly where it came from. The galaxy is faint and small (though not a dwarf galaxy), so it has not been detected in previous images of the region. Leloudas tells Inverse, “We have to wait for the transient light to fade away and then go back and see if we can find the host galaxy.”
Daniel Perley, an astronomer at Liverpool John Moores University, notes that it’s not clear why, as a star is pulled apart by a black hole with millions of times the mass of the sun and shredded into its accreditation disk, a black hole will occasionally expel a fraction of that material at almost the speed of light. “The mass of the star and black hole might be involved, but the majority of tidal disruption events don’t have a relativistic jet.” Says Leloudas, “these are rare occasions where we can see them turn on and off.”
What’s next — With only three of these jets sighted so far, it’s a difficult question to resolve. But, Perley tells Inverse, “Now we have a roadmap for doing it.” The Vera Rubin Observatory’s Legacy Survey of Space and Time in northern Chile is expected to come online in 2024, and, like the Zwicky Transient Facility, will be taking pictures of the whole night sky in optical wavelengths. Andreoni adds, “we didn’t know until today what to look for if one wanted to study jetted tidal disruption events in the optical, because there was no example before.”
Because the LSST will have a huge field of view – 3.5 degrees, which may not sound like much, but is about 8 times the diameter of the moon in the night sky – and an extremely deep depth of field, Leloudas says, “everything is going to be scaled up” in number and distance. “It will find thousands of tidal disruption events, while now we’re in the tens.”
This won’t be without its issues. Not only will it be difficult to study these optical events in other wavelengths, because “you need an even bigger telescope to take spectra of things that are far, far away,” as Leloudas adds, it will be difficult to determine which of the events are actually significant. Says Tanvir, “the concern might be that rare but unknown events could be missed because we just don’t have enough follow-up capacity ot chase everything.”
Andreoni adds, “the hope is that one more type of weirdo or weird source… will unveil a whole population of these events for which we now have just one example.”