BURSTS OF MYSTERY
The source of mysterious bursts of radio waves has been confounding scientists for a decade. Now we’re QDOO\ PDNLQJ VRPH SURJUHVV ZULWHV Govert Schilling
Decades after their discovery, scientists are finally beginning to understand fast radio bursts.
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.
The upshot: 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. “There’s still an awful lot of work ahead of us,” observes 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 they, 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.
Closing in on the signal
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 six billion lightyears. 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
“If FRB 121102 is repeating, you can just keep an eye on the suspect part of the sky to localise a new outburst in real time”
catastrophic events, even though they couldn’t be 100 per cent 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 last year, 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 disc 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.
So yes, there’s still a lot of work to do. Astrophysicists look forward to the inauguration this year of the CHIME radio telescope in Canada, which may detect a few dozen FRBs per day. Meanwhile, Dutch and South African astronomers are about to deploy the 65cm optical MeerLICHT telescope at the South African Astronomical Observatory in Sutherland, a very promising instrument in the search for the true nature of FRBs.
MeerLICHT will automatically and continuously scan the same region of sky as the South African radio interferometer MeerKAT, some 250km to the north. As soon as MeerKAT happens to catch a fast radio burst, a possible optical afterglow will be captured by MeerLICHT, enabling detailed follow-up observations. “No one has ever tried this approach before,” says project manager Steven Bloemen of Radboud University in Nijmegen in the Netherlands. “It may revolutionise the field.”
The FRB afterglow is similar to that left behind by a short gamma-ray burst, which are created when binary neutron stars collapse into one another
The arrival of the ‘dispersed’ Lorimer burst; inset: the same burst once dedispersion has been applied
Fast radio bursts have been something of an enigma, lasting only for a few milliseconds and seemingly occurring at random
Some believe that FRBs may be caused by intermittent explosions on rapidly spinning neutron stars, but there is no consensus so far