For more than a decade, researchers have sought to solve the puzzle of fast radio bursts, millisecond chirps of radio waves that seem to appear at random in the sky, likely from unknown sources millions or even billions of light-years away. Theorists have many ideas for what causes them, including stellar flares, cataclysmic mergers of neutron stars and evaporating black holes. They have so many ideas, in fact, that the theories for FRB sources have long outnumbered the recorded bursts. That paucity of observed bursts reflects the hardest part of the FRB puzzle: guessing where to point a telescope to catch them as they happen.
Now, however, astronomers know where to look to reliably see at least one: a patch of sky about one tenth the size of the full moon in the direction of the constellation Auriga. That is where researchers using the Arecibo radio telescope in Puerto Rico detected 11 FRBs over the past four years, all apparently originating from the same mysterious astrophysical source. The findings are reported in the journal Nature (Scientific American is part of Springer Nature). “The fact it repeats rules out—for this object anyway—any of the models that are just one-offs, whether they involve mergers or evaporating black holes or something else,” says study co-author James Cordes, an astronomer at Cornell University. Instead, Cordes says, the more probable culprit is some sort of powerful outburst from a rotating neutron star.
The trouble is, no neutron stars have ever been seen behaving quite as strangely as the one Cordes guesses might be behind these FRBs. After detecting an initial burst from the source in 2012, the team looked again in May and June last year, finding 10 more bursts sporadically scattered through the new data. Two flashed into the telescope on one day and eight on another. The bursts varied wildly in their intensities and arrival times, with only about a minute separating the two closest together. Whatever process is producing the bursts does not appear to follow the periodic pattern that would be expected for a regularly rotating neutron star.
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Cordes suggests that perhaps the bursts’ strange properties could be due to a neutron star in a tight orbit around a black hole, where the effects of immense gravitational forces could mask any periodicity to bursts coming from the neutron star. “Nature is more clever than theorists, and I think most of us would hope there are new, exotic phenomena to be identified here, but we have to be careful to not let expectations trump reality,” Cordes says. “I am a minimalist—if you can explain something by known phenomena, you should.”
Whatever the source is, it must be far away, researchers say. The calling card of an FRB’s cosmic journey is a smearing-out of its wavelengths, produced when it passes through clouds of plasma between its source and Earth. The greater the smearing, or “dispersion measure,” the more plasma it has passed through and the farther away its source is likely to be. “The dispersion measure for the bursts we saw has a value three times higher than what is likely to be produced by our galaxy,” says Slavko Bogdanov, a study co-author at Columbia University. “If the bursts came from within our galaxy, they’d run out of material to pass through, so the source must be beyond the Milky Way’s edge.”
Just how far the FRB source is beyond our galaxy’s edge “is the million-dollar question,” Bogdanov says. Presumably, most of an FRB’s dispersion comes from interactions with the tenuous plasma of the intergalactic medium, a vast cosmic web that stretches through the mostly empty space between galaxies. But plasma within an FRB’s host galaxy should contribute to the dispersion, too, muddying the distance estimates for any given FRB. Cordes estimates the source of Arecibo’s repeating FRB is perhaps a few hundreds of millions of light-years away but acknowledges it could also be in a particularly plasma-rich galaxy much closer. Although the team has stared into the repeating FRB’s patch of sky using several telescopes, they have yet to locate a likely host galaxy.
Last week a separate study in Nature reported another result seemingly in conflict with the Arecibo observations. As detailed by Evan Keane of the international Square Kilometer Array Organization and colleagues, this separate study suggested some fraction of FRBs occur billions rather than millions of light-years away. Such FRBs would probably be one-time events, produced by astrophysical mergers so energetic they destroy their source objects, prohibiting any recurrence. Using radio telescopes in Australia and optical telescopes in Hawaii, Keane and his colleagues detected an FRB and linked its fading afterglow to a host galaxy some six billion light-years from Earth. That’s so far off that no conceivable neutron star outburst short of a pair colliding in a one-time merger would be bright enough to be seen from Earth. Such a result would establish at least some FRBs as bona fide cosmic probes, capable of mapping the otherwise-invisible cosmic web across billions of light-years. Catalogued in sufficient numbers, those far-reaching FRBs could be used to create a sort of “tomographic scan” of the universe.
“That’s the great hope,” Cordes says. “And consequently some people are biased in favor of that turning out to be the case.” Within days of the publication of the Keane et al study, however, Peter Williams and Edo Berger of the Harvard–Smithsonian Center for Astrophysics released an independent analysis casting grave doubts on the claimed linkage between the FRB and the galaxy billions of light-years away.
Further certainty about the source of that particular FRB, if it ever comes at all, is likely to only emerge through more detailed follow-up studies using other facilities, such as the Jansky Very Large Array (VLA) in New Mexico or the yet-to-be-built Square Kilometer Array in Australia and South Africa. The VLA and other facilities will also prove crucial to further studies of the repeating Arecibo FRB as researchers seek to detect more bursts and better understand the nature of the source.
Despite the apparent divide between researchers as to whether FRBs are immensely energetic one-time events from the edges of the observable universe or smaller, repeating ones from much closer by, there is plenty of room in the cosmos for both phenomena to coexist.
“People used to think supernovae and gamma-ray bursts were all the same, then we learned they come from different types of events,” Bogdanov says. “It could be the case that FRBs come from at least two distinct varieties of natural events. So the next obvious step is to find more of them, and to stare at the others we’ve already found and see if they repeat. That would help us learn which ones are which!”