Why ultra-powerful radio bursts are the most perplexing mystery in astronomy - Part 2
Elizabeth Gibney, Nature | 28 June 2016
Strange signals are bombarding Earth. But where are they coming from?
Bursts of inspiration
But that still leaves the question of what the FRBs actually are. The extreme brevity of the signal, just 5 milliseconds, implied that the source must be a compact object no more than a few hundred kilometres across — a stellar-mass black hole, perhaps, or a neutron star, the compact core left over by a supernova. And the fact that Earth-based telescopes can detect the FRBs at all means that this compact source somehow puts out an immense amount of energy. But that still leaves a long list of candidates, from merging black holes to flares on magnetars: rare neutron stars with fields hundreds of millions of billions of times stronger than the Sun's.
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An important clue arrived earlier this year when Spitler's team reported that at least one FRB source repeats: data from Arecibo revealed a flurry of bursts over two months, some spaced just minutes apart7. That behaviour has been confirmed by the Green Bank telescope, which detects signals in a different frequency band8. Until then, each of the observed FRBs had been a one-off event, which hinted at cataclysmic explosions, or collisions in which the sources were destroyed. But a repeating FRB implies the existence of a source that survives the pulse event, says Petroff. And for that reason, she says, “I would assume it would be something to do with a neutron star” — one of the few known objects that can emit a pulse without self-destructing.
Spitler agrees. As an example, she points to the Crab nebula: the result of a supernova explosion that was observed from Earth in 1054 and left behind a rapidly spinning pulsar surrounded by glowing gas. The Crab pulsar occasionally releases extremely bright and narrow radio flares, Spitler says. And if this nebula were in a distant galaxy and hugely boosted in energy, its emissions would look like FRBs.
Bizarre star could host a neutron star in its core
If one source repeats, Spitler says, the simplest interpretation is that they all do, but that other telescopes haven't been sensitive enough — or lucky enough — to see the additional signals. Yet others think that perhaps only some are repeating. “I wouldn't be surprised if we end up with two or three populations,” says Petroff.
A long way home
Another crucial question is how far away the FRBs are. The 20 bursts seen so far seem to be scattered randomly around the sky rather than being concentrated in the plane of the Galaxy, which suggests that their sources lie beyond the borders of the Milky Way.
And yet to Avi Loeb, a physicist at Harvard University, such vast distances imply an implausibly large energy output. “If you want the burst to repeat, you won't be able to destroy the source — therefore, it cannot release too much energy,” he says. “That puts a limit on how far away you can put it.” Perhaps, he says, the FRB sources are neutron stars in our own Galaxy, and the dispersion is mostly the result of still unknown electron clouds that blanket them.
But others suggest that such a dense cloud in the Galaxy should be visible in other wavelengths. At the California Institute of Technology (Caltech) in Pasadena, astrophysicist Shri Kulkarni has scoured data from several telescopes for a galactic source and turned up nothing9. Kulkarni, who directs Caltech's optical observatories, initially argued for galactic FRBs, and even made a US$1,000 bet on it with astronomer Paul Groot of Radboud University Nijmegen in the Netherlands. Now, he finds the evidence for extragalactic FRBs to be overwhelming, and has agreed to settle the bet — grudgingly. “I think I will pay him in $1 bills,” he says.
Still, Kulkarni hasn't ruled out the possibility that the FRB sources lie in galaxies that are perhaps a billion light years away, rather than many billions. Such a distance would still require at least some of the signal dispersion to come from electron clouds in the host galaxy, he says. But closer FRBs would not have to be so energetic. “It takes them from being amazingly exotic, to just exotic,” he says.
The answer could mean a great deal to observers. If the FRB signals have travelled through local plasma clouds, they could give weather reports from neighbouring galaxies. But if they are truly cosmological — coming from halfway across the visible Universe — they could solve a long-standing cosmic mystery.
For decades, astronomers have known from observations of the early Universe that the cosmos should contain more everyday matter — the kind made up of electrons, protons and neutrons — than exists in the visible stars and galaxies. They suspect that it lies in the cold intergalactic medium, where it is effectively invisible. But now, for the first time, the dispersion of the FRB signals could enable them to measure the medium's density in any given direction. “Then, we have essentially a surgical device to do intergalactic tomography,” says Kulkarni.
Rapid-fire detection
First, however, astronomers have to find a lot more FRBs and pin down their locations. “Until then, we are stumbling in the dark,” says Berger.
One way to accomplish that is to extract the FRBs from radio-telescope data in real time, so that scientists at other observatories can observe the bursts in multiple wavelengths. Since last year, the Parkes team has been doing this by boosting the observatory's in-house computing power, and scientists at Arecibo hope to follow suit this year. In February, the strategy seemed to be paying off when an independent team followed up within two hours of an FRB's detection at Parkes. The team tentatively pinpointed the burst to a specific galaxy almost 6 billion light years away. Further observations cast doubt on that interpretation. But even so, says Lorimer, the method is sound and may pay off in the future.
The Australia Telescope Compact Array, in New South Wales, which helped to identify the location of a fast radio burst.
Others observers are putting their hopes in new telescopes. In 2014, astrophysicist Victoria Kaspi at McGill University in Montreal, Canada, submitted a proposal to adapt CHIME, which was originally designed to map the expansion of the Universe in its early years. “It became clear very quickly that it would be a fantastic FRB instrument,” says Kaspi. Although dish telescopes such as Arecibo can be highly sensitive, they observe only a single, tiny patch of sky at a time. CHIME, by contrast, consists of four 100-metre-long half-pipes dotted with antennas that can monitor much bigger stretches of sky in long lines. After undergoing testing and debugging, CHIME should see its first FRBs sometime next year, says Kaspi, ultimately finding more than a dozen per day.
In Hoskinstown, Australia, meanwhile, Bailes and his colleagues are refurbishing the 1960s-vintage Molonglo Observatory Synthesis Telescope, turning it into an FRB observatory with a single half-pipe 16 times longer than CHIME's, although one-quarter as wide. The team has already found three as-yet-unpublished FRBs with the facility working at only about 20% of its final sensitivity, says Bailes.
Another strategy for locating the FRB sources is to work with existing facilities such as the Very Large Array: an 'interferometer' that uses the time difference between signals from 27 radio telescopes spaced across 36 kilometres of grassland near Socorro, New Mexico, to create a single, high-resolution image. Sometime in the next year or so, says Lorimer, the array could detect an FRB and locate its home galaxy. “Ultimately, that could settle a lot of arguments and bets,” he says.
Kulkarni, meanwhile, is leading two projects. The first uses ten 5-metre-wide dishes in an array that can see and locate only super-bright FRBs, but that makes up for its low sensitivity by peering at a huge swathe of sky. The second takes the principle to the extreme, using 2 antennas spaced at observatories 450 kilometres apart that will see only the very brightest FRBs, but that are able to examine half the sky at once. That would enable it to catch the rare FRBs that presumably exist within our own Galaxy, but whose extreme brightness existing telescopes are not designed to see. “Most facilities would just discount it as interference,” says Kulkarni.
If FRBs do turn out to come from cosmological distances, says Loeb, their identification would be a major breakthrough, potentially unravelling a new class of source that could be used to probe the Universe's missing matter. But then, he says, FRBs could also be something that no one has thought of yet: “Nature is much more imaginative than we are.”
