Unexplained, short radio bursts from outer space have puzzled scientists since they were first detected nearly a decade ago.
The elusive flashes — known as fast radio bursts, or FRBs — are extremely powerful and last only a few milliseconds. The way their frequencies are dispersed suggests they traveled from far outside our galaxy. About 18 have been detected to date. They've been called the "most perplexing mystery in astronomy."
Scientists are still grappling with why these bursts happen. But researchers have now pinpointed the source of one series of the FRBs — to a dwarf galaxy billions of light-years away from Earth.
And locating the source of the mystery bursts could hold clues to what is causing them, according to Shami Chatterjee, an astrophysicist at Cornell University. He's the lead author in a paper recently published in Nature.
Let's talk quickly about one burning question: Could the source be aliens? Probably not, Chatterjee tells The Two-Way. "Never say never, but we don't think so. We can view this with physics that we know and understand." More-likely explanations involve a neutron star or an active galactic nucleus, though there are a slew of possibilities, he says.
The story of this particular burst, called FRB 121102, took a wild turn when scientists found that its signal repeated. This immediately eliminated a number of theories about why it was happening — for example, it couldn't be two neutron stars colliding. "Because we know right away that it can't be explosive. Whatever it is, has to survive this radio flash," Chatterjee adds.
The equipment used to detect FRBs is able to see only a tiny patch of the sky at any moment. The discovery that FRB 121102 repeated suggested that it was a good direction to point the detection equipment. "If you go fishing in this spot in the sky, you might be more likely to get lucky than in other random spots in the sky," Chatterjee says.
In 2015, the team began using an interferometer — in this case, a network of 27 radio dishes called the Very Large Array that's in New Mexico — which is capable of much higher resolution detection than other readings.
"It was a pretty intensive observational and computational challenge," Chatterjee says. The interferometer captured data at 200 frames per second from this patch of sky, he says, resulting in a terabyte of data every hour that put a major strain on archival and computational resources.
During the first 10 hours of recording this sliver of sky, they found nothing. They recorded 40 more hours — again, no bursts. Frustrated, the team also enlisted the Arecibo Observatory in Puerto Rico.
This time, they got lucky. They captured nine radio images of bursts, allowing them to "pin it down to an absolutely tiny patch of the sky" for the first time.
In that patch of the constellation Auriga lies a dwarf galaxy a fraction of the size of our own, Chatterjee says, some 2.5 to 3 billion light-years away. It's worth noting that the sheer distance of the flashes' origin makes "catching it in the act" very relative, since the event that caused it happened billions of years ago.
So what was that event? Chatterjee says there are many theories. It could be originating from an active galactic nucleus, which emits FRBs as blobs of plasma drift into its jets and are vaporized. Or it could be originating from a newborn magnetar — a neutron star with an extremely strong magnetic field — that is "emitting these giant pulses as it spins." It also could be the interaction between a magnetar and a black hole, or many other possibilities, he adds.
A crucial question now, Chatterjee says, is whether all FRBs repeat like this one, or whether there are types that don't repeat.
"They're probably the same thing and we haven't been lucky enough to observe the other ones repeating," he says. "But if not, hey, great, nature's given us two fantastic mysteries instead of one fantastic mystery."
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