Searching for distant signals

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Duncan Lorimer wasn’t looking for an eruption of radio waves from another galaxy. He and his student David Narkevic were mining old data from Australia’s Parkes Radio Telescope for oddly behaving pulsars, the rapidly spinning cores of dead massive stars. Instead, they found a strange burst of radio noise recorded in 2001 that appeared to originate well beyond one of the satellite galaxies that orbit the Milky Way.

The signal was so intense that it briefly overwhelmed the telescope. “It took me a while to come to terms with it,” says Lorimer, an astrophysicist at West Virginia University in Morgantown. “I knew it was unusual, but I just wasn’t able to grasp the whole gravity of the situation.” In 2007, Lorimer wrote in Science that the burst “represents an entirely new phenomenon.”

Just one signal was a curiosity. But in 2011, astronomer Evan Keane, who was at the Jodrell Bank Centre for Astrophysics in Manchester, England, reported a second one, also in archival data from Parkes.

Thornton’s discovery set the field ablaze. The radio bursts appeared to be the calling card of some exotic, cataclysmic event outside of the Milky Way. Intriguingly, if these signals were truly racing through the space between galaxies, then they may have encountered — and be able to tell the story of — half the missing matter in the universe (see sidebar).

The more astronomers look, the more FRBs they find. Eight fast radio bursts have been reported so far, with another dozen or so yet to be published. This new cosmic mystery is the kind of thing astronomers find irresistible: an exploration of the unknown, and a reminder of how little we know about the universe.

“There’s a race on,” says Dale Frail, assistant director for the Very Large Array in Socorro, N.M., “a friendly scientific race.” Astronomers are dusting off old telescopes and refurbishing others. Radio observatories around the globe are scanning the skies to figure out what these bursts are — and where they come from. 

“People are finding them, but they don’t know what they mean,” says Keith Bannister, an astronomer at CSIRO Astronomy and Space Science in Marsfield, Australia. “There are big arguments about whether they’re real or not, whether they’re very local or very distant.” Speculation about the origin of fast radio bursts runs from the mundane to the exotic. “There are probably at least twice as many theories as there are FRBs at the moment,” Lorimer says.

Of blitzars and magnetars

With the scant data that are available, astronomers are narrowing down the possibilities. Each FRB arrives as a single pulse, which leads people to think that the bursts are probably caused by eruptions or implosions. While FRBs are powerful enough to be seen from other galaxies, they’re not as energetic as an exploding star. And they last for only a few milliseconds, so the source has to be relatively compact, roughly as wide as Texas. Keane, now at Swinburne University of Technology in Hawthorn, Australia, favors two ideas, both involving dead stars.

When a massive star explodes, its core stays behind. Within the core, a struggle ensues between the crushing force of gravity and the outward push of subatomic particles. If the core weighs more than a few suns, gravity wins, and the core collapses into a black hole. If the core is smaller, neutrons keep gravity in check, and the core survives as a neutron star.

If the core is rotating very fast, says Keane, it’s possible to be heavy and still be a neutron star. Pushing back against gravity, the rotation keeps the core stable. But everything that spins eventually slows down. When the overweight neutron star slows enough, it implodes in an event known as a blitzar. Theoretically, the collapse could generate a radio pulse that looks like an FRB.

Or the bursts could be eruptions from a magnetar, Keane says. Magnetars are neutron stars with a magnetic field around 1 million billion times as strong as Earth’s. Astronomers have seen similar bursts from magnetars in our galaxy — the brightest one hit Earth in 2004 and blinded the Swift space telescope, even though it was pointed in a different direction at the time (SN: 2/26/05, p. 132).

There are ways to tell the two apart: Magnetars repeat, blitzars do not. “You can only destroy a neutron star and make a black hole once,” says Keane. Magnetars also radiate light at much higher frequencies, such as gamma rays; blitzars, on the other hand, would probably emit only low-frequency radio light. For now, the more common magnetars seem the more likely suspect of the two.

To figure out if blitzars or magnetars — or something else entirely — make sense, astronomers need to know how far away these bursts are. Estimates of distances depend on knowing what the light encountered on its way to Earth. As light crosses space, it plows through plasma, gas clouds where electrons roam free. How easily the light navigates around the electrons depends on the light’s frequency. High-frequency light passes through with relative ease; low frequencies take a bit longer.

If something blasts out radio waves, all the frequencies leave as a group, but they don’t arrive together at their destination. A radio telescope will see the higher radio frequencies before the lower frequencies. The result, if you could hear it, might sound like a slide whistle. The delay between arrival times indicates how much stuff the light stumbled upon on its way to Earth.

Astronomers can use the number of electrons encountered by the radio waves as a proxy for estimating the distance that the light traveled. Count up the electrons: If the light ran into more electrons than astronomers expect exist between Earth and the edge of the Milky Way, then the signal must originate outside of our galaxy. Astronomers only have theoretical calculations to estimate how many electrons should be floating about in space, but the five bursts detected by Lorimer and Thornton met far more electrons than can reasonably fit within the Milky Way.

Keane’s burst, however, is different. That FRB passed through the plane of our galaxy and shot through the Scutum star cloud, a window in the interstellar dust. The gap allowed CSIRO’s Bannister and colleagues to peer deep into the galaxy and directly measure how many electrons sit between Earth and Keane’s burst. They reported in the May 1 Monthly Notices of the Royal Astronomical Society that this burst most likely originated from within the Milky Way. If they’re right, then the Keane burst might be an erratic pulsar or an annihilating black hole. It may also mean that FRBs are two distinct but similar phenomena: one from our galaxy and one from farther afield.

Here or there

Then again, the bursts may not be coming from space at all.

After the first FRB was reported, Sarah Burke-Spolaor, a Caltech astrophysicist, and colleagues went digging through old Parkes data. They found another 16 bursts that closely resemble the other FRBs with one key exception: They appeared to originate from within Earth’s atmosphere, based on the way they hit the telescope. Another group recently saw five similar bursts at the Bleien Radio Observatory near Zurich. In both cases, the bursts typically arrived late in the morning, which means these local radio blips appear to be tied to Earth’s daily rhythm. Even more so than FRBs, their origin has scientists baffled.

Burke-Spolaor named these atmospheric bursts perytons after a mythological winged elk that casts the shadow of a human, appearing as something it is not. Perytons make some astronomers hesitant about FRBs. Shri Kulkarni, also from Caltech, wonders if FRBs are just perytons that are very high in the atmosphere. An FRB beyond about 20 kilometers above Parkes, he says, may look like it came from another galaxy.

Laura Spitler’s recent detection may help. The astrophysicist from the Max Planck Institute for Radio Astronomy in Bonn, Germany, found a seventh FRB, reported in the August 1 Astrophysical Journal, within 2012 data from the Arecibo radio telescope in Puerto Rico. At 305 meters across, Arecibo is the largest operating radio dish on the planet. Based on Arecibo’s enormous size, Kulkarni says this burst must be at least 400 kilometers away, about the altitude of the International Space Station and beyond the bulk of Earth’s atmosphere.

Spitler’s burst also puts to rest any concern that FRBs are a quirk of the Parkes telescope. But it doesn’t resolve the issue of whether they arise in the Milky Way or not. Like Keane’s burst, Spitler’s signal sits in the plane of the galaxy. “My thought is that we’re currently looking at three different populations,” says Burke-Spolaor: bursts within Earth’s atmosphere, within the Milky Way and in other galaxies. 

Familiar territory

Astronomers have been down this road before. To ferret out the true nature of FRBs, scientists just need to look at how they solved the mystery of gamma­-ray bursts.

In the 1960s, the U.S. Air Force launched a family of satellites to scrutinize the skies for gamma rays — the highest-energy, highest-frequency light — produced by illicit nuclear weapons tests. The satellites recorded 16 flashes, only none were coming from Earth and no one knew what they were. Almost three decades later, NASA launched the Compton Gamma Ray Observatory to look for more. By the end of the mission, Compton tallied about 2,700 bursts coming from every direction in the sky. Even then, astronomers still weren’t sure exactly what produced the flashes or how far the light had traveled.

Finally, in 1997, the Italian-Dutch BeppoSAX satellite detected a gamma-ray burst and hours later caught an associated X-ray flash. Within days, telescopes on the ground saw a fading glow of visible light that seemed to sit on top of a galaxy. Astronomers quickly measured the spectrum of the visible light, which they used to calculate the distance to the burst: at least 9 billion light-years from Earth.

Astronomers now know that there are at least two subsets of gamma-ray bursts: explosions of massive stars and collisions between neutron stars in other galaxies. The key to cracking the mystery was the real-time detection and rapid follow-up from telescopes at other frequencies of light, which let researchers pinpoint the galaxy where the burst originated. The same should work for FRBs, Bannister says: Quickly and accurately locate a burst “and then you hit it with as many telescopes as you can, and see what you find.”

Gearing up

Right now, the locations of FRBs in the sky are fuzzy. The bursts that Parkes sees could be coming from any one of tens of thousands of galaxies. But what it lacks in precision, Parkes makes up for in size. It’s a large telescope — 64 meters across. It can collect a lot of light and detect fainter bursts. And with 13 receivers each looking at a different patch of sky, it can cast a relatively wide net.

Keane is using Parkes for a project called SUPERB — the SUrvey for Pulsars and Extragalactic Radio Bursts. (“You’ve got to have a catchy acronym,” he says.) SUPERB is trying to do for FRBs what BeppoSAX did for gamma-ray bursts: quickly alert other observatories when a burst goes off. The infrastructure to do this already exists, thanks to the gamma-ray community. As soon as SUPERB sees a potential FRB, it sends an electronic alert to other telescopes, which then reposition themselves to look for a fading afterglow.

One of those telescopes is the Molonglo Observatory Synthesis Telescope, a mile-long radio antenna in Australia. Matthew Bailes, also at Swinburne, is overseeing an effort to refurbish and update the 47-year-old facility with modern electronics, including a direct fiber-optic link to Parkes. When SUPERB sees a burst, it will send a virtual and immediate heads-up to Molonglo.

Other radio observatories also want in. Astrophysicist Casey Law of the University of California, Berkeley is hunting for FRBs with the Very Large Array — 27 radio dishes separated by up to 36 kilometers across the Plains of San Agustin in New Mexico. By precisely noting the arrival times of radio signals at all 27 dishes, astronomers can pinpoint a burst to a single galaxy halfway across the visible universe, Law says. Another group is searching with the Very Long Baseline Array, a facility that takes the VLA concept and extends it to 10 radio telescopes scattered from Hawaii to the Virgin Islands. The telescopes are virtually linked to create an antenna more than half the width of Earth. Whereas the VLA can narrow the origin of an FRB down to a specific galaxy, the VLBA might see where within that galaxy the burst came from.

Despite predicted FRB rates as high as 10,000 per day, the VLA and VLBA teams don’t expect to find many bursts — one or two at most. The VLBA, for example, can see only about a full moon’s worth of sky at one time, and it takes a lot of moons to cover the sky. But astronomers need only one event to know if a burst is coming from another galaxy.

If there are more FRBs out there, radio astronomers will find them. Many other observatories across the United States, Australia and elsewhere are gearing up to catch them as quickly as possible. Both Arecibo and West Virginia’s Green Bank Telescope are building new equipment, including high-end video game microprocessors to do the number crunching, so they can conduct their own searches. And new facilities like the Murchison Widefield Array in Western Australia are hunting over large swaths of sky at very low frequencies. Each telescope has its strengths and weaknesses. Some see more of the sky but can see only the brightest bursts. Others are extremely accurate but, with a narrow field of view, take a long time to find just one.

“It’s been a roller coaster for everyone,” says Burke-Spolaor, “intrigue, curiosity and also frustration.” The only way to sort out the confusion is to look, and the entire community is stepping up. “Everywhere I go,” Keane says, “people are saying, ‘Hey, I have a telescope. I’d like to help.’ ”

If current estimates are correct, an FRB goes off somewhere in the sky once every 10 seconds. We’ve just become aware because we now have the tools to see them. “It’s like there’s this incredible symphony going on above our heads,” Lorimer says, “and we just can’t quite figure it out yet.” He pauses. “But we will.”

Weighing the universe

Bright signals coming from across the universe can help scientists probe the space between galaxies. And that might help solve a cosmological conundrum: Where is half the matter in the universe?

But when astronomers look around the present universe, about half the atoms seem to have gone missing.

Astronomers think the missing matter is in the intergalactic medium, or IGM, an expanse of plasma that fills the darkness between galaxies. Unfortunately, the IGM is sparse, which makes it difficult to study. A room filled with the plasma would contain just two electrons — and nothing else. At such low densities, the plasma barely emits any light. And light is one of the few tools astronomers have for studying the universe.

Fast radio bursts may help. If they originate in other galaxies, they must pass through the IGM. By measuring a burst, says Jean-Pierre Macquart, an astrophysicist at the Curtin Institute of Radio Astronomy in Perth, Australia, you can account for every particle the light encountered en route to Earth. If astronomers can find a lot of FRBs, he adds, they might solve a problem they’ve been struggling with for years.