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 610 | NATURE | VOL 534 | 30 JUNE 2016 | CLARIFIED 4 JULY 2016 © 2016 Macmillan Publishers Limited. All rights reserved FEATURE NEWS 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.