What are fast radio bursts?
The source of mysterious bursts of radio waves has been confounding scientists for a decade. Now we’re finally making some progress.
Thousands of times each day, an ultra-brief, intense burst of radio waves is generated somewhere in the Universe, and astronomers have frustratingly few clues as to their origin.
Duncan Lorimer, a radio astronomer at West Virginia University in the US, had no idea what to expect when his student David Narkevic re-analysed observations from July 2001, carried out with the 64m Parkes radio telescope in Australia.
Lorimer certainly couldn’t imagine that Narkevic’s 2007 analysis would open up a new field in astrophysics that is still mystifying scientists after 10 years.
“There’s still an awful lot of work ahead of us,” says Lorimer.
The ‘Lorimer burst’ of 24 July 2001 was no freak event. Occasionally, other Parkes observations also caught similar fast radio bursts (FRBs), lasting a few milliseconds at most and occurring randomly in the sky.
Taking the extremely narrow field of view of the radio telescope into account, it was easy to calculate that these mysterious explosions must in fact be very frequent.
But what are fast radio bursts, and where do they come from?
If you don’t know the location of an FRB (either within our own Milky Way or far beyond), you can’t deduce the energies of the outburst.
It could be a relatively minor explosion on the surface of a nearby dwarf star, or a titanic event at the edge of the observable Universe.
Astronomers had only one clue: all FRBs display a relatively strong ‘dispersion’, which means that lower-frequency waves lag behind higher-frequency ones.
This effect is caused by the waves passing through a tenuous gas of charged particles.
High dispersion means lots of intervening particles, corresponding to large distances. But no one could be completely sure.
To complicate matters, a single-dish radio telescope like Parkes has a relatively low spatial resolution on the sky, so the locations of the bursts weren’t very precisely known.
That made it impossible to carry out follow-up observations with larger telescopes.
This only changed in April 2015 when a team led by Evan Keane of the Square Kilometre Array Organisation pointed the Australia Telescope Compact Array (ATCA) at a region of sky in the constellation of Canis Major, where Parkes had discovered a fast radio burst just a few hours earlier.
On the basis of the Parkes detection, the sky position of FRB 150814 couldn’t be determined to a precision better than some 15 arcminutes (half the width of the full Moon).
But ATCA is an interferometer array with much sharper vision, and in the 0.25º ‘error box’ of FRB 150814, Keane and his colleagues found a slowly fading radio source, located in a galaxy at a distance of 6 billion lightyears.
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The afterglow of FRB 150418 was reminiscent of the afterglow of a short gamma-ray burst, which are the result of neutron star collisions in remote galaxies.
In late February 2016, Keane’s team wrote in Nature that FRBs are most likely one-off catastrophic events, even though they couldn’t be 100% sure about the association between the FRB and the ATCA radio source.
But just a week later, Nature published another paper by Canadian, American and Dutch radio astronomers that came to a very different conclusion.
Using the 305m radio telescope at Arecibo Observatory in Puerto Rico, Paul Scholz of McGill University in Montreal and his colleagues had discovered a repeating fast radio burst: FRB 121102 in Auriga also displayed outbursts in May and June 2015.
So whatever was causing these bursts didn’t destroy its source. As Lorimer says: “It’s basically impossible for a cataclysmic event to produce repeating bursts.”
Now, the hunt was really on. If FRB 121102 is repeating every now and then, you can just keep an eye on the suspect part of the sky with a large interferometer to localise a new outburst in real time.
Patience paid off, eventually. In September 2016, both the Very Large Array in New Mexico and the European VLBI Network succeeded in tracing down the source of the bursts to a small, inconspicuous dwarf galaxy at a distance of some 2.5 billion lightyears.
“It’s an observational breakthrough,” said NASA astrophysicist and gamma-ray burst expert Neil Gehrels when he learned about the results, just before his untimely death in February 2017.
From the known distance, astronomers could now calculate the burst’s energy – about as much in one millisecond as the Sun pours out in 24 hours.
Canadian-Dutch radio astronomer Jason Hessels now believes FRBs are occasional explosions from extremely rapidly spinning, highly magnetised neutron stars.
Others think that the repeating bursts may occur in the accretion disk surrounding the black hole that probably lurks in the dwarf galaxy’s core.
Meanwhile, Lorimer is not so sure that there’s just one type of fast radio burst – after all, FRB 121102 is the only one known to repeat so far.
“My guess is that there are multiple classes,” he says.
The peryton problem
Around the time that fast radio bursts were discovered, astronomers also came across a very similar type of millisecond signal, albeit with a much different pulse profile.
They looked very artificial – for instance, they were equally bright at all wavelengths – but their true origin remained unknown for a long time.
Parkes radio astronomers called them perytons, after a mythological creature that is half stag, half bird.
In 2015 the peryton puzzle was evenutally solved. It was found that perytons were generated when the door of the microwave oven in the astronomers’ kitchen was opened prematurely.
No new cosmic riddle: just impatient scientists and technicians on site who believed
that their lunch was ready!
Unfortunately, the confusion around perytons did slow down the study of genuine fast radio bursts, says Duncan Lorimer, who described the first FRB in 2007.
“It made it harder to get our research funded.”
The Canadian Hydrogen Intensity Mapping Experiment (CHIME) in Penticton, British Columbia, is a discovery machine for fast radio bursts.
CHIME, run by four Canadian institutes, is an all-sky observatory, designed to map the distribution of neutral hydrogen in the early Universe, so its main goal is cosmology.
But given its extremely large field of view, it is expected to detect dozens of relatively bright fast radio bursts per day.
The ‘digital telescope’ has no moving parts at all. It consists of four cylindrical ‘half-pipes’, oriented north to south, and measuring 20x100m.
The telescope’s orientation with respect to the sky changes as a result of Earth’s rotation, which makes it possible to use interferometry to create detailed hydrogen maps.
According to radio astronomer Vicky Kaspi of McGill University in Montreal, CHIME could be an excellent FRB detector, despite observing at lower frequencies than Parkes, Arecibo or the Very Large Array.
This article originally appeared in the June 2017 issue of BBC Sky at Night Magazine.