Astronomers Zero In On Source of Mysterious Fast Radio Burst

The distant radio flashes of FRBs are yet to be fully explained.

Michael Byrne

Michael Byrne

Image: NRAO

Fast radio bursts are among the coolest, strangest phenomena in the universe. They are much as they sound—sudden millisecond flashes of radio waves of unknown origin, most likely extragalactic. Some say they're alien communiques, others say they're radio artifacts from human equipment right here in Earth's orbital backyard. Most astronomers assume that they're the product of some so-far poorly understood or unknown cosmological process, such as a starquake, mutant black hole, or pulsar.

FRBs also happen to be quite rare, or they are given our current detection abilities. Since first discovering the phenomenon a decade ago, we've bagged only 16 of them. Six of those detections have been in the past nine or so months, and almost all observations have come courtesy of the Parkes Radio Telescope in Australia—though we can thank the Green Bank Telescope for this one. The bursts are usually discovered as artifacts within existing astronomical data.

Now, for the first time, astronomers have tracked an FRB back to its origins or very nearly so. In a paper published in the current Nature, a group of researchers led by the University of British Columbia's Kiyoshi Masui describe observations made of FRB 110523 that indicate its source—whatever that may be—is most likely within a star-forming nebula or the remnant of a supernova. (An open-access version of the paper is hosted at arXiv.)

The finding is as much an achievement of software data-mining as it is astronomy as we normally think of it. Masui, with help from Jonathan Sievers of the University of KwaZulu-Natal in South Africa, developed a new method of parsing large amounts of radio-wave data in relatively short amounts of time.

Green Bank Telescope. Image: Jiuguang Wang

"The original algorithm goes way back to 1970," Sievers told me in a Skype interview. "It was developed by Joe Taylor, who won a Nobel Prize for detecting the first binary pulsar and testing general relativity with it." FRBs were still unknown in 1970, but pulsar hunting and FRB hunting have a lot in common.

"If you rework [the algorithm] to be optimized for current computers and pull a few tweaks that people hadn't yet pulled, it works quite well," Sievers explained. "Nowadays with a nice consumer-grade graphic card, for the parameter space used for FRB searches, you can run 400 separate telescopes in real-time and do the search. It's really fast."

They found the FRB in question, but with a curious additional property known as Faraday Rotation. Basically, as electromagnetic waves (such as radio waves) pass through some magnetized medium, some wave components are more affected than others (imagine a radio wave as being simultaneously polarized in different directions) and so the effect is that the signal becomes twisted up like a corkscrew.

The meaning of this for our FRB is that we can say that it passed through two different "screens"—regions of ionized gas—on its way to Earth. The strongest of these screens is located very near the burst's source, within a hundred thousand light-years anyhow, which indicates that it either originated within a nebula or very near a galactic center.

So, we know more about where the FRB came from, but do we know what it came from? The intensity of the incoming signal provides an upper-limit on the size of its source, simply for the reason that if the source were larger the signal would be more spread out and diffuse. The astronomers then eliminated the possibility of the burst originating as a flare from some star, which eliminates any possible sources within the Milky Way.

This leaves a few possibilities then: magnetar starquakes, delayed formation of black holes after core-collapse supernovae, and pulsar giant pulses. The latter idea seems especially promising.

"Pulsars are strange beasts," Sievers said. "[Their timing] is incredibly regular, but their brightness is all over the place. If I tell you the brightness of a pulse, you'd get almost no information about what the brightness of the next pulse will be. It could be 10 times brighter; 20 times brighter; 50 times brighter; or it could drop out completely."

"Occasionally you get some rare events where a pulsar will put out one pulse that's much brighter than it normally is," he said. "It could be hugely brighter. That's one possibility: that these are coming from pulsars that just happen to put out a one-off incredibly bright pulse."

The next task in unraveling the FRB mystery is in observing one with more than just a single telescope. So far, our views have been from lone perspectives, but there are many more radio telescopes scanning the sky and collecting reams and reams of data just waiting to be computed. Now we have a tool to do so.