In A Far-Off Galaxy, A Clue To What's Causing Strange Bursts Of Radio Waves

Feb 24, 2016
Originally published on March 9, 2016 6:07 am

One of the most puzzling astronomical discoveries of the past decade has just gotten a little bit clearer. Astronomers still don't know what's producing the brief, powerful bursts of radio waves they've been detecting, but for the first time, they've been able to see where one of them is coming from.

Astronomers first detected these so-called fast radio bursts in 2007. Until now, all 16 FRBs that have been reported have been found by combing through archival data.

But Evan Keane, an astronomer with the Square Kilometer Array Organization, says he and his colleagues wanted to catch one in the act. That way, they could point other telescopes in the direction of the FRB to search for clues about its origin.

So they piped data from Australia's CSIRO Parkes radio telescope into a supercomputer at the Swinburne University of Technology in Melbourne, Australia, so that it could sift through the information in real time. On April 18, 2015, they got a hit. It was only the 17th FRB ever reported.

"I was in South Africa when it happened," Keane says. He'd been in a long engineering meeting the day before and had been planning to sleep in that Saturday morning. "But of course my phone started going crazy in the morning" with people calling to say the computer had found an FRB. They called it FRB 150418, for the day it was found.

Keane and his colleagues immediately alerted a network of telescopes they had assembled around the world to help pinpoint and characterize an FRB if and when they found one.

The Australia Telescope Compact Array at the Paul Wild Observatory was able to observe the radio "afterglow" from FRB 150418. ATCA is an array of six 22-meter antennas used for radio astronomy, and it was able to localize the source with a thousand times the accuracy of the Parkes telescope.

Then, the Japanese Subaru telescope took up the search. Subaru is a large optical telescope located on Mauna Kea in Hawaii. It was able to see the patch of sky where ATCA showed FRB 150418 had come from.

"There's only one thing there," Keane says, "and it's a galaxy, an elliptical galaxy."

An elliptical galaxy 6 billion light-years away.

Unfortunately, Keane says, knowing that doesn't explain what's generating the massive pulse of radio energy.

But knowing how far away that object is lets you do some extremely interesting calculations. That's because the different frequencies that make up the radio burst don't all arrive at the exact same time. The longer frequencies are delayed.

"And the reason for that delay is the stuff that the signal has gone through — the particles and dust in the intervening space," Keane says. And by measuring the delay, you can measure how many particles there are between us and the galaxy.

And knowing how many particles there are between here and a galaxy 6 billion light-years away gives you an estimate of how much that slice (or cylinder, if you'd prefer) of the universe weighs.

Although what's generating these FRBs is still a mystery, the new discovery gives some hints. Keane says the galaxy where the FRB originated comprises mostly older stars. "Our conclusion [is] that FRB 150418 is likely to be from a one-off event in an older stellar population," he and his colleagues write in the journal Nature, in a paper describing the find.

Keane's organization, the Square Kilometer Array Organization, is designing a giant radio telescope that should be able to detect lots more of these FRBs. Maybe enough to figure out what's making them.

Before you ask, there's no good evidence that FRBs are being generated by extraterrestrials.

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Over the past decade, astronomers have been seeing these weird, powerful, but very brief bursts of radio waves out in space. No one knows what's going on. But now for the first time, astronomers have been able to pinpoint where one of these bursts came from. Here's NPR's science correspondent Joe Palca.

JOE PALCA, BYLINE: OK, let's get this out of the way right at the start. We're not talking about aliens generating radio signals - at least, I'm not, and neither were the scientists I spoke with. But these fast radio bursts, or FRBs, are still fascinating because they're so darn powerful and so brief. And because they're so brief, until now, no one has seen one happen in real time. They were found by combing through data weeks or months after the actual event occurred. Catching one in the act would provide additional valuable clues about them. So Evan Keane and his colleagues set out to trap one. They piped the data from the Parkes Radio Telescope in Australia into a supercomputer, and let the supercomputer scan the data in real time. On April 18 last year, they got a hit - only the seventeenth FRB ever reported.

EVAN KEANE: I was in South Africa when it happened. And I was planning to have - you know, to sleep in. But of course, my phone started going crazy in the morning, waking me up, telling me there was an FRB.

PALCA: They had a hit. Now they had to pinpoint it. Keane had a network of telescopes standing by.

KEANE: We immediately triggered other telescopes and tell them there's an FRB over here. Can you observe it? When can you observe this?

PALCA: First, another radio telescope in Australia was able to see the radio afterglow from the FRB and narrow in on the patch of sky where the burst had come from. Then, Japanese colleagues pointed a large, optical telescope in Hawaii, called Subaru, at that spot to see what was there.

KEANE: There's only one thing there, and it's a galaxy - an elliptical galaxy.

PALCA: ...An elliptical galaxy 6 billion light-years away. Of course, it's still not possible to say what's inside the elliptical galaxy that's generating the massive pulse of radio energy. But it does help to know how far the radio waves have traveled, since that tells you something about the power needed to generate them. Keane works with something called the Square Kilometer Array Organization, a giant radio telescope still in the planning stages, that should be able to detect lots more of these FRB - maybe enough to figure out what's making them. The current research appears in the journal Nature. Joe Palca, NPR News. Transcript provided by NPR, Copyright NPR.