You'd think they'd be hard to miss: unimaginably powerful bursts of cosmic radiation, occurring perhaps as often as a thousand times every day, so bright that they can blast our radio telescopes from billions of light years away.
But fast radio bursts (FRBs) went undetected until 2007, and despite a decade and a half of investigation, remain one of the most enticing mysteries in astrophysics. Recent studies are providing new and promising hints about their origins while at the same time illustrating just why these cosmic firecrackers are so confounding in the first place.
When I first started hearing about FRBs in seminars, the big question was not so much “What astrophysical source is causing this?” but rather “Are we sure this wasn’t just a blip in the machine?”
After all, whatever it was, it looked pretty suspicious. An FRB is a burst of radio radiation that lasts around a millisecond and spreads out in frequency in a way that looks an awful lot like a blip from a pulsar (a rapidly spinning core of a collapsed massive star known as a neutron star that’s left after a supernova explosion).
But here's the thing: they don't come from any known pulsar, don't repeat like a pulsar, and are apparently far more powerful than any pulsar pulse we've ever seen.
To make matters worse, for years, there was only one telescope – the Parkes Observatory in Australia – that had seen any FRBs at all. The debate got even more heated when it was discovered that some fraction of FRB-like bursts seen by Parkes were not from astronomical sources.
Called ‘perytons’, these bursts had always been suspected to be of terrestrial origin, but the case was closed after some clever detective work led by astronomer Dr Emily Petroff.
She and her colleagues showed that perytons were strongly correlated with local lunchtime and were, in fact, leakage of radiation from the observatory microwave when the door was opened too early. Could FRBs also be some kind of technological mistake?
It eventually became clear that FRBs are definitely coming from the very distant Universe. More radio telescopes were configured to be able to record very short radio bursts and the detection rate started to skyrocket.
Those bursts were coming from all over the sky, which hinted that they didn’t originate in our galaxy (where they would have been concentrated toward the disk or centre). In the first decade after their discovery, theorists produced a huge number of papers describing possible origins for the bursts, including powerful supernovae, exploding primordial black holes, and cosmic strings, to name just a few.
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The plot thickened in 2012 when one FRB was found to be repeating. This ruled out any origin, at least for that burst, that required complete destruction, like a supernova or other cataclysmic explosion. Soon, more bursts were found to repeat, though still only a small minority, and mostly in confusing, irregular intervals.
As more bursts were discovered, evidence was building that FRBs might be related to unusually powerful magnetars: spinning neutron stars with extremely strong magnetic fields.
Bolstering this hypothesis was the discovery of a strong, short burst of radiation from a known magnetar in our own galactic backyard.
It wasn’t as powerful as the observed fast radio bursts, but it looked like it might be something in the same class of event – a missing link between FRBs and the kinds of radio outbursts we routinely see from pulsars.
In just the last few months, even more evidence in favour of at least some FRBs having a magnetar origin has come from a study of more than a dozen relatively nearby FRBs detected with the CHIME radio telescope in British Columbia, Canada (which was originally constructed to study hydrogen in our Galaxy but has turned into an FRB-detecting powerhouse).
The researchers were able to determine that all the FRBs in their sample came from spiral galaxies (which tend to have a lot of star formation) rather than in elliptical ones (which tend to have mostly ageing stars).
They argue that their study provides evidence that for the bulk of FRBs, whatever makes them is probably produced in a core-collapsed supernova: the explosive death of a short-lived massive star (preferentially found in locations of active star formation) that can leave a magnetar as a remnant.
Competing origin proposals either don’t occur often enough to account for the number of FRBs, or would not be so likely to be found in only spiral galaxies. Of course, it’s still possible – and perhaps likely – that FRBs are more than one kind of thing.
Some have suggested that collisions of old stars could produce magnetars in elliptical galaxies, which could explain why some FRBs have been found there.
Astronomers will just have to keep gathering clues, looking for suggestive patterns in the data, and eagerly awaiting observational upgrades that will allow us to pinpoint the local environments of FRBs.
Whatever they turn out to be, fast radio bursts are an excellent example of the fact that in science, looking at the Universe in a new and different way almost always results in finding something amazing that no one thought to look for at all.
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