Monday, January 26, 2015

Inside the Global Effort to Track Mysterious Space Radio Signals

Astronomers are building a global collaboration to identify recently discovered radio blips seemingly originating from deep space. Called "fast radio bursts," or FRBs for short, these enigmatic cosmic signals have so far confounded astronomers, and are the subject of a growing effort to track and observe them.

An artist's rendition of the Parkes telescope observing a cosmic radio signal.
Image: Swinburne Astronomy Productions.

A lot of questions surround the origins of these strange signals, and astronomers are stepping up efforts around the world to identify where they're coming from. Each radio chirp lasts just a few milliseconds and were only identified after scientists went back and reviewed years-old data and found them hiding in plain sight.

"We've known about these FRBs for a while but we don’t know a lot about them," said Emily Petroff, a PhD candidate at Swinburne University of Technology and organizer of the collaboration.

A newly formed international collaboration to track and observe these signals had its first dry run in May. Scientists at Australia's Parkes radio telescope got lucky and for the first time saw the signature radio flash while the telescope was pointed at it. Within seconds their computer identified the quick flash and the team got word out to telescopes around to world to look for the source of the mysterious signal.

"It was really exciting for everybody that we finally got one," said Emily Petroff.

Over the next several days, telescopes from Hawaii to Chile covering everything from radio waves to x-rays, were trained on the patch of sky where the signal originated. Unfortunately, on this first try the team wasn't able to see anything that could explain the radio signals.

"We weren't quite done setting up the collaboration as we could have been when it happened… [so] it wash't as fast a response as it could have been," Petroff said. "We’re hoping once everything is set up and running smoothly…  we'll know that we've found it within about 10 seconds."

She added that the plan is to have other telescopes pointed at the region "within minutes" of the identification of the signal. For May's event, notification went out via email to teams working at the other telescopes and the fastest turnaround time was just under seven hours.

Though not as fast as it could have been, the list of telescopes trained on the spot in space was impressive. First on the scene was the Australia Telescope Compact Array, looking for any signs also in the radio band. NASA's SWIFT space telescope soon followed and looked for any residual X-ray signals. Soon after came the Swope Telescope in Chile, the Gamma-Ray Burst Optical/Near-Infrared Detector, the Keck Telescope and the Nordic Optical Telescope and six others around the world.

However, even though the followup observations didn't turn up anything, scientists working on the project say that as the collaboration gets under way, response times will improve and with some luck they might be able to catch some FRB trace in the future.

"I would deem myself very lucky if it worked the very first time," said Daniele Malesani, a physicist at the University of Copenhagan.

Scientists first identified FRBs as a potential new astronomical phenomena in 2006, but the early claims were met with some initial skepticism. Milliseconds long radio blips could easily have been the result of faulty equipment. However in 2013, Dan Thornton, an astronomer at the University of Manchester in England was able to find four in old data taken from a range of telescopes, proving there was something out there. But astronomers are still not sure what.

"We really don’t know the source of these things yet," said Jim Cordes, an astronomer at Cornell unconnected to FRB hunting team. "It's a great mystery because the further away it is, the more energy it needs for us to be able to see it."

With the little data they have, scientists are able to only make a few inferences about the source of these FRBs. Because they're so short and seem to leave no traces behind the teams can knock a few items off the list of possible sources.

"It helps us rule out a couple of different candidates,"Petroff said. "It does rule out things like local supernovae or these things like long gamma ray bursts."

It's also unclear how close they are. Most of the astronomers seem to think that they are likely coming from sources outside our galaxy. Because interstellar space is not quite a perfect vacuum, interceding stellar dust causes radio signals to slightly deteriorate in a predictable way over great distances. Astronomers say that based on how the the FRB's radio signal tapers off, its likely that they're originating outside the galaxy.

The teams are optimistic that they'll see more soon. Though FRBs have only recently been observed, they're not rare occurrences.

"It seems they are extremely common over the entire sky," Malesani said.

The estimates are that there are between 3,000 and 5,000 such flashes every day somewhere overhead. But they're easy to miss, they each only last a few milliseconds at most and get overlooked while researchers look for the repeating signals from pulsars.

"The reason so few have been discovered, is that no one was searching the old data for these things until recently," Petroff said. She added also that radio telescopes are only able to look at very small sections of the sky at a time, limiting their ability to catch one-off events like these.

However because they are so common, that also helps to narrow the possibilities of what could cause them.

"Whatever these objects are, you need a lot of them, they can't be rare objects," Cordes said. He added that he liked the idea that they came somehow from neutron stars, which are the dense leftovers of a massive star and there seems to be a lot nearby. "Just from a numbers point of view, it's a plausible kind of thing."

There are other reasons to think that the sources might be neutron stars. The recent observations by the Parkes telescope also found that the radio signals were slightly circularly polarized, hinting that there might be a powerful magnetic field at their source, which are a defining feature of neutron stars and magnetars.

Though scientists are still in the speculative phases, Petroff added that it seemed that the strongest possibilities had to do with neutron star phenomena. She said it could be either the burst from a gigantic solar flare or a neutron star's swan song as it collapses under its own gravity.

"It's possible one of these mega-flares caused these fast radio bursts," she said. "It's possible it would be [the] death signal of a neutron star as it becomes a black hole."


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