In November 2012, a powerful blast of radio waves hit the Earth. This event—called a fast radio burst—intrigued scientists with its extreme energy, faraway origin, and seeming lack of clues about whatever mysterious astrophysical engine had blasted the waves into space.
But unlike the few fast radio bursts that had been observed before, this one was destined to be more than a one-hit wonder.
In early 2015, another blast of radio waves was fired by the same unknown engine. And another. And then another, and another, until finally, over a period of three hours, a total of ten additional bursts emerged from the same location on the sky.
It’s the first time astronomers have caught a fast radio burst in the act of repeating, and—as reported this week in Nature—the observations provide crucial clues about the origin of these bursts, which have stumped astronomers for nearly a decade.
“I just plain love this discovery,” says astronomer Emily Petroff of the Netherlands Institute for Radio Astronomy. “The observations and data are very solid. A super fascinating result and definitely a really big deal.”
Until now, fast radio bursts were generally thought to be the result of cataclysmic, one-time astrophysical events such as supernovas, stars collapsing into black holes, or colliding stars. That’s because no one had ever seen a burst go off more than once. But if the same source continues firing radio waves into space, that rules out a bunch of ideas. It’s not like a supernova can put the band back together and go on a revival tour.
“If it happened four years ago and it’s happening again now, it’s not some star that collapsed or black holes that merged,” says study co-author Scott Ransom of the National Radio Astronomy Observatory. “This is probably a very young, very fast spinning magnetar, in some local-ish galaxy, that occasionally goes into semi-outbursty form.”
Blasts from the Past
First detected in 2007, fast radio bursts have been a stubborn, messy puzzle to solve. Including these 10 new ones, roughly two dozen of the bursts have been observed, and many appear to be coming from billions of light-years away. Yet they last for just thousandths of a second. For years, scientists have argued over whether the bursts were really coming from outside the Milky Way galaxy—and whether the bursts were even real or were artifacts produced by telescopes on Earth.
As more observations trickled in, astronomers gradually began to agree that yes, the bursts are real, and yes, they do appear to be coming from far away. But their origins eluded capture.
Recently, an important piece of puzzle snapped into place when a team of scientists studying the large-scale structure of the universe serendipitously snared a burst that contained a new, essential piece of information: In addition to coming from very far away, the burst appeared to have traveled through a highly magnetized region of space, suggesting that perhaps a flaring, extremely magnetic neutron star—a magnetar—could be spinning away at its origin.
It’s a story that matches these new observations, and it could be a window into the world of at least one burst engine.
“A Minor Point of Interest”
First observed in 2012, the repeater burst is known colloquially as the Spitler burst, after Max Planck postdoc Laura Spitler, who discovered it while sifting through data from a pulsar survey at the Arecibo Observatory. Soon after that discovery, Spitler and her colleagues re-targeted the burst, known less colloquially as FRB 121102, and searched for repeats. They got nothing—just like the teams at Australia’s Parkes Observatory that had spent dozens and dozens of hours staring at bursts and waiting for them to go off again.
In mid-2015, Spitler and her colleagues decided to re-observe FRB 121102, knowing they might be on a futile hunt. Over three hours, Arecibo aimed its powerful eye at the burst’s spot on the sky. Then it was time to wait for supercomputers to process all the data. Later, when McGill University graduate student Paul Scholz took a look at the processed observations, he got a surprise. Well, ten of them.
“Right away, I knew that this was a big new part of the story of fast radio bursts,” Scholz says. “It kind of changes everything.”
As one does, Scholz decided to have a little fun with his team. He sent an email announcing the discovery with the subject line, “A minor point of interest regarding the Spitler burst.” In it, he revealed the ten additional bursts Arecibo had detected.
"I can just imagine how exciting a moment it must have been for their team to see the first repeat!” Petroff says. “This rules out loads of progenitor models. Cataclysmic models are out the window for this source. Whether that means they're out or not for the whole population remains to be seen.”
Curiously, the burst rate isn’t stable and is changing dramatically over time. Six bursts appeared within about 10 minutes, and several others were separated by weeks. “That means the object itself is changing somehow,” Ransom says. The team’s best guess is still that the bursts are flares from a magnetar, and that it’s somewhere on the order of a few hundred million light-years away. But giant pulses from extragalactic pulsars could also be plausible.
Duncan Lorimer, who discovered the first burst in 2007, suspects that more follow-up observations could reveal a pattern in the burst’s timing. “Sooner or later, I think it’s going to reveal its periodicity,” says Lorimer, who’s at West Virginia University. But more importantly, he notes, the observations should crush any lingering doubts about fast radio bursts being real.
“This one is repeating from the same patch of the sky, but at completely different epochs,” he says. “That’s just a really positive result from their paper. But of course, the conundrum it throws up then is, do all of them repeat? Or just some fraction?”
Multiple Burst Engines?
The story the Spitler burst tells is different than the one told by a fast radio burst described last week in Nature. That report, which has since been challenged by follow-up observations, describes a fast radio burst that may have originated from the catastrophic collision and merger of two neutron stars—an event that left a radio afterglow in its wake.
Assuming both observations are real, it’s not too surprising that there may be multiple ways to cook a fast radio burst. “It is still possible that fast radio bursts, at least some fraction of them, could be due to catastrophic events, and that maybe there’s two classes,” Ransom says.
Though two species of fast radio bursts might seem to defy the principle that the simplest explanation is usually the best, it wouldn’t be the first time the cosmos has decided to get a little tricky. In the 1960s, when gamma-ray bursts were first detected, scientists didn’t know what to make of them. By the 1990s, observations revealed that these blindingly bright blasts of gamma-rays could be produced by both supernovas and colliding neutron stars—and that the resulting species of bursts varied slightly from one another.
In other words, there’s no reason in the universe why fast radio bursts should only come from one source. The transient, flickering radio sky is not particularly well understood, and as astronomy has shown us for millennia: Each time we see the sky in a new light, unexplained, unimaginably complex phenomena emerge.