Ultra-Powerful Signals Known As Fast Radio Bursts Are Bombarding Earth

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MYSTERY IN THE HEAVENS Ultra-powerful signals known as fast radio bursts are bombarding Earth. But where are they coming from?

BY ELIZABETH GIBNEY

o astronomer had ever seen anything an astrophysicist at West Virginia University in easily result from mobile-phone signals, stray like it. No theorist had predicted it. Yet Morgantown, saw this object erupt only once, radar probes, strange weather phenomena there it was — a 5-millisecond radio and with more power than any known pulsar. and instrumental glitches. Wider acceptance Nburst that had arrived on 24 August 2001 from He began to realize the significance of the of what is now known as the Lorimer burst an unknown source seemingly billions of light discovery1 only after carefully going over the came only in the past few years, after observers ENGLAND WAYNE years away. data with his former adviser, Matthew Bailes, an working at Parkes and other telescopes spotted “It was so bright, we couldn’t just dismiss it,” astrophysicist at Swinburne University of Tech- similar signals. Today, the 2001 event is rec- says Duncan Lorimer, who co-discovered the nology in Melbourne, Australia. If the source ognized as the first in a new and exceedingly signal1 in 2007 while working on archived data really was as far away as it seemed, then for a peculiar class of sources known as fast radio from the Parkes radio telescope in New South few milliseconds it had flared with the power of bursts (FRBs) — one of the most perplexing Wales, Australia. “But we didn’t really know 500 million Suns. “We became convinced it was mysteries in astronomy. what to do with it.” something quite remarkable,” he says. Whatever these objects are, recent observa- Such fleeting radio bursts usually came But when no more bursts appeared, initial tions suggest that they are common, with one from pulsars — furiously rotating neutron excitement turned to doubt. Radio astrono- flashing in the sky as often as every 10 sec- stars whose radiation sweeps by Earth with the mers have learnt to be sceptical of mysterious onds2. Yet they still defy explanation. Theorists regularity of a lighthouse beam. But Lorimer, spikes in their detectors: the events can all too have proposed sources such as evaporating

The Parkes telescope black holes, colliding space — not least because all the FRBs thus far black hole, perhaps, or a neutron star, the com- in Australia detected neutron stars and had been seen by one team using one telescope. pact core left over by a supernova. And the fact the first fast radio enormous magnetic “I was desperate for someone else to find them that Earth-based telescopes can detect the FRBs burst in 2001. eruptions. But even somewhere else,” says Bailes. at all means that this compact source somehow the best model fails In 2014, his wish was finally granted. A puts out an immense amount of energy. But to account for all the observations, says Edo team led by astronomer Laura Spitler at the that still leaves a long list of candidates, from Berger, an astronomer at Harvard University in Max Planck Institute for Radio Astronomy merging black holes to flares on magnetars: rare Cambridge, Massachusetts, who describes the in Bonn, Germany, published their observa- neutron stars with fields hundreds of millions of situation as “a lot of swirling confusion”. tions of a burst at the Arecibo Observatory in billions of times stronger than the Sun’s. Clarity may come soon, however. Telescopes Puerto Rico5. “I was ridiculously overjoyed,” An important clue arrived earlier this year around the world are being adapted to look for says Bailes. when Spitler’s team reported that at least one the mysterious bursts. One of them, the Cana- The Arecibo discovery convinced most FRB source repeats: data from Arecibo revealed dian Hydrogen Intensity Mapping Experiment people that FRBs were the real deal, says Emily a flurry of bursts over two months, some spaced (CHIME) near Penticton in British Columbia, Petroff, who is now an astrophysicist at the just minutes apart7. That behaviour has been should see as many as a dozen FRBs per day Netherlands Institute for Radio Astronomy in confirmed by the Green Bank telescope, which when it comes online by the end of 2017. Dwingeloo. Yet, as long as the Burke-Spolaor detects signals in a different frequency band8. “This area is set to explode,” says Bailes. signals went unexplained, they cast a shadow Until then, each of the observed FRBs had been of doubt. “At any talks I would give,” says a one-off event, which hinted at cataclysmic CURIOUSER AND CURIOUSER Petroff, “someone would say, ‘But what about explosions, or collisions in which the sources Astronomers might have had more confi- were destroyed. But a repeating FRB implies dence in the Lorimer burst initially had it not the existence of a source that survives the pulse been for a discovery in 2010 by Sarah Burke- event, says Petroff. And for that reason, she Spolaor, who was then finishing her astrophys- “THERE’S NO says, “I would assume it would be something ics PhD at Swinburne. Burke-Spolaor, now an to do with a neutron star” — one of the few astronomer at the US National Radio Astron- known objects that can emit a pulse without omy Observatory in Socorro, New Mexico, was WAY THAT’S A self-destructing. trawling through old Parkes data in search of Spitler agrees. As an example, she points more bursts when she turned up 16 signals that to the Crab nebula: the result of a supernova shook everyone’s confidence in the original3. MICROWAVE OVEN.” explosion that was observed from Earth in In most ways, these signals looked remark- 1054 and left behind a rapidly spinning pulsar ably similar to the Lorimer event. They, too, perytons?’” So in 2015, while still a graduate surrounded by glowing gas. The Crab pulsar showed ‘dispersion’, meaning that high- student at Swinburne, she led a hunt to track occasionally releases extremely bright and nar- frequency waves appeared in the detectors down the source of perytons once and for all. row radio flares, Spitler says. And if this nebula a few hundred milliseconds before the low- First, Petroff and her team used the upgraded were in a distant galaxy and hugely boosted frequency ones. This dispersion effect was Parkes detector to pinpoint when the bursts in energy, its emissions would look like FRBs. the most important piece of evidence con- were happening: at lunchtime. “Immediately I If one source repeats, Spitler says, the simplest vincing Lorimer and Bailes that the original thought, ‘This isn’t weather’,” says Petroff. Then interpretation is that they all do, but that other burst came from well beyond our Galaxy. came another peryton at a suspiciously famil- telescopes haven’t been sensitive enough — or Interstellar electrons in clouds of ionized iar radio frequency, which led the team to run lucky enough — to see the additional signals. gas are known to interact more with low- experiments in the staff kitchen. Perytons, they Yet others think that perhaps only some are frequency waves than with high-frequency discovered, were the result of scientists opening repeating. “I wouldn’t be surprised if we end ones, which delays the low-frequency waves’ the microwave oven mid-flow. But the Lorimer up with two or three populations,” says Petroff. arrival at Earth ever so slightly, and stretches event was in the clear: records showed that at the signal (see ‘Flight delays’). The delay in the the time of the burst, the telescope had been A LONG WAY HOME Lorimer burst was so extensive that the wave pointed in a direction that would have blocked Another crucial question is how far away the had to have travelled through a lot of matter — any microwave signal from the kitchen6. FRBs are. The 20 bursts seen so far seem to be much more than is in our Galaxy. “So then I worried, maybe they’ve just got scattered randomly around the sky rather than Unfortunately for Lorimer and Bailes’ peace a different brand of microwave at Arecibo,” being concentrated in the plane of the Galaxy, of mind, Burke-Spolaor’s signals also showed says Bailes, whose team at Parkes had, by then, which suggests that their sources lie beyond a crucial difference from the original: they racked up 14 separate bursts. He did not relax the borders of the Milky Way. seemed to pour in from everywhere, not just completely until later in 2015, when a burst was And yet to Avi Loeb, a physicist at Harvard from where the telescope was pointing. Dubbed spotted at a third facility — the Green Bank Tel- University, such vast distances imply an implau- perytons, after a mythical winged creature that escope in West Virginia. That burst had another sibly large energy output. “If you want the burst casts a human shadow, these bursts could have quality that supported an extraterrestrial ori- to repeat, you won’t be able to destroy the source been caused by lightning, or some human-made gin: its waves were rotated in a spiral pattern — therefore, it cannot release too much energy,” source. But they were not extraterrestrial. — which results from passing through a mag- he says. “That puts a limit on how far away you Lorimer decided to postpone his research netic field — and were scattered as if they had can put it.” Perhaps, he says, the FRB sources into FRBs for a while. “I didn’t yet have ten- emerged from a dense medium. “There’s no way are neutron stars in our own Galaxy, and the ure,” he says, “so I had to go back and do more that’s a microwave oven,” Bailes told himself. dispersion is mostly the result of still unknown mainstream projects, just to keep my research electron clouds that blanket them. moving.” Bailes and his team kept going, and BURSTS OF INSPIRATION But others suggest that such a dense cloud upgraded the Parkes detector’s time and fre- But that still leaves the question of what the in the Galaxy should be visible in other wave- quency resolution. In 2013, they turned up four FRBs actually are. The extreme brevity of the lengths. At the California Institute of Tech- new FRB candidates that resembled the Lorimer signal, just 5 milliseconds, implied that the nology (Caltech) in Pasadena, astrophysicist burst4. But some outsiders remained scepti- source must be a compact object no more than a Shri Kulkarni has scoured data from several cal that the signals were really coming from few hundred kilometres across — a stellar-mass telescopes for a galactic source and turned up

nothing9. Kulkarni, who directs Caltech’s next year, says Kaspi, ultimately finding optical observatories, initially argued for FLIGHT DELAYS more than a dozen per day. galactic FRBs, and even made a US$1,000 Astronomers are not sure what causes fast radio bursts (FRBs). In Hoskinstown, Australia, meanwhile, But as the waves reach Earth, low-frequency ones lag behind

453–456 (2016). bet on it with astronomer Paul Groot of Bailes and his colleagues are refurbishing Radboud University Nijmegen in the high-frequency ones. The extent of this delay suggests that the the 1960s-vintage Molonglo Observatory 530,

signals have travelled through intergalactic space for potentially Netherlands. Now, he finds the evidence billions of light years. Synthesis Telescope, turning it into an SOURCE: FIG. 1 IN KEANE, for extragalactic FRBs to be overwhelm- FRB observatory with a single half-pipe ing, and has agreed to settle the bet — 16 times longer than CHIME’s, although NATURE; NATURE; SOURCE Frequency Event ET AL. NATURE ET grudgingly. “I think I will pay him in $1 An unknown event High Low one-quarter as wide. The team has already E. F. E. F. bills,” he says. emits a huge burst found three as-yet-unpublished FRBs of radio waves over Still, Kulkarni hasn’t ruled out the a range of 5 ms with the facility working at only about NIK SPENCER/ possibility that the FRB sources lie in gal- frequencies 20% of its final sensitivity, says Bailes. axies that are perhaps a billion light years simultaneously. Another strategy for locating the FRB away, rather than many billions. Such a sources is to work with existing facilities distance would still require at least some such as the Very Large Array: an ‘inter- of the signal dispersion to come from Electron clouds ferometer’ that uses the time difference electron clouds in the host galaxy, he between the between signals from 27 radio telescopes says. But closer FRBs would not have to galaxies interact spaced across 36 kilometres of grassland with the waves, be so energetic. “It takes them from being stretching and near Socorro, New Mexico, to create a amazingly exotic, to just exotic,” he says. slowing the lower single, high-resolution image. Sometime Interstellar The answer could mean a great deal to frequencies more in the next year or so, says Lorimer, the strongly than the clouds observers. If the FRB signals have trav- higher ones. array could detect an FRB and locate its elled through local plasma clouds, they home galaxy. “Ultimately, that could set- could give weather reports from neigh- tle a lot of arguments and bets,” he says. bouring galaxies. But if they are truly Kulkarni, meanwhile, is leading two cosmological — coming from halfway projects. The first uses ten 5-metre- across the visible Universe — they could wide dishes in an array that can see and solve a long-standing cosmic mystery. locate only super-bright FRBs, but that For decades, astronomers have known makes up for its low sensitivity by peer- from observations of the early Universe ing at a huge swathe of sky. The second that the cosmos should contain more A telescope on takes the principle to the extreme, using everyday matter — the kind made up of Earth measures 2 antennas spaced at observatories the delay and electrons, protons and neutrons — than stretch, enabling 450 kilometres apart that will see only exists in the visible stars and galaxies. astronomers to the very brightest FRBs, but that are able They suspect that it lies in the cold inter- estimate how far to examine half the sky at once. That the signals have galactic medium, where it is effectively come. would enable it to catch the rare FRBs invisible. But now, for the first time, that presumably exist within our own the dispersion of the FRB signals could Galaxy, but whose extreme brightness enable them to measure the medium’s existing telescopes are not designed to density in any given direction. “Then, we see. “Most facilities would just discount have essentially a surgical device to do SIGNAL it as interference,” says Kulkarni. intergalactic tomography,” says Kulkarni. If FRBs do turn out to come from The signal is lost in the noise until the cosmological distances, says Loeb, their RAPID-FIRE DETECTION telescope’s output identification would be a major break- First, however, astronomers have to find is separated into through, potentially unravelling a new frequency bands. a lot more FRBs and pin down their loca- This reveals a class of source that could be used to probe tions. “Until then, we are stumbling in cascade of peaks the Universe’s missing matter. But then, the dark,” says Berger. that corresponds he says, FRBs could also be something to the dispersion of One way to accomplish that is to the burst. (gigahertz) Frequency that no one has thought of yet: “Nature extract the FRBs from radio-telescope is much more imaginative than we are.” ■ data in real time, so that scientists at other Time (ms) observatories can observe the bursts in Elizabeth Gibney is a reporter for multiple wavelengths. Since last year, the Nature in London. Parkes team has been doing this by boosting the Kaspi at McGill University in Montreal, Canada, 1. Lorimer, D. R., Bailes, M., McLaughlin, M. A., observatory’s in-house computing power, and submitted a proposal to adapt CHIME, which Narkevic, D. J. & Crawford, F. Science 318, 777–780 scientists at Arecibo hope to follow suit this year. was originally designed to map the expansion (2007). In February, the strategy seemed to be paying off of the Universe in its early years. “It became 2. Champion, D. J. et al. Mon. Not. R. Astron. Soc. Lett. 460, L30–L34 (2016). when an independent team followed up within clear very quickly that it would be a fantastic 3. Burke-Spolaor, S., Bailes, M., Ekers, R., Macquart, two hours of an FRB’s detection at Parkes. The FRB instrument,” says Kaspi. Although dish J.-P. & Crawford, F. III Astrophys. J. 727, 18 (2011). team tentatively pinpointed the burst to a spe- telescopes such as Arecibo can be highly sensi- 4. Thornton, D. et al. Science 341, 53–56 (2013). 5. Spitler, L. G. et al. Astrophys. J. 790, 101 (2014). cific galaxy almost 6 billion light years away. tive, they observe only a single, tiny patch of sky 6. Petroff, E. et al. Mon. Not. R. Astron. Soc. 451, Further observations cast doubt on that inter- at a time. CHIME, by contrast, consists of four 3933–3940 (2015). pretation. But even so, says Lorimer, the method 100-metre-long half-pipes dotted with antennas 7. Spitler, L. G. et al. Nature 531, 202–205 (2016). 8. Scholz, P. et al. Preprint at http://arxiv.org/ is sound and may pay off in the future. that can monitor much bigger stretches of sky in abs/1603.08880 (2016). Others observers are putting their hopes in long lines. After undergoing testing and debug- 9. Kulkarni, S. R., Ofek, E. O. & Neill, J. D. Preprint at new telescopes. In 2014, astrophysicist Victoria ging, CHIME should see its first FRBs sometime http://arxiv.org/abs/1511.09137 (2015).

CLARIFICATION In the News Feature ‘Mystery in the heavens’ (Nature 534, 610–612; 2016), the discussion of the initial radio burst meant to say that over the course of just a few milliseconds, the source’s output matched that of 500 million Suns in the same time period.