Tracing the origins of fast radio bursts

Radio astronomers monitoring the sky for unusual events have found a fast radio burst (FRB) in the constellation Vulpecula, 30,000 light-years from Earth in the Milky Way. It is the first time such an event has been observed in our galaxy.

Now called FRB 200428, it occurred on 28 April and is reported by two independent teams: one using an instrument in Canada and the other an array of detectors in the southwestern US.

FRBs are enormous blasts of radio energy that in a few milliseconds can broadcast as much energy in radio waves as the monthly output of the Sun in all forms, combined.

By intergalactic standards, FRB 200428 is a pipsqueak – a factor of 30 weaker than the weakest known extragalactic FRB, and 1000 times less powerful than the average one, says Christopher Bochenek from California Institute of Technology, who was a member of one of the teams.

But still, “this is the most luminous radio source ever detected in our own galaxy”, adds Daniele Michilli, an astrophysicist with Canada’s McGill Space Institute and a member of the other.

It is also close enough, Bochenek says, for astronomers to be able to link it to a gamma-ray source known as SGR 1935+2154. “This is the first FRB that has come from a known object”.

SGR 1935+2154 is a magnetar. “That’s a type of neutron star whose magnetic field is so strong they squish atoms into pencil-like shapes.”

Such stars are known to produce brief, intense flares that emit rapid bursts of gamma rays and X-rays. In fact, Bochenek’s team found, FRB 200428 was accompanied by just such an outburst of X-rays.

Artist’s impression of a magnetar, showing complex magnetic field structure and beamed emission, imagined as following a crust cracking episode. Credit: McGill University

Magnetars have long been regarded as a top candidate for producing FRBs, says Bing Zhang, an astrophysicist at the University of Nevada, US. “People have been speculating this for many years.”

But, he adds, his team was working with a giant radio telescope in China and happened to be watching the magnetar from 16 April to 29 April but saw no signs of FRBs accompanying 29 other gamma-ray bursts emitted by it during that time interval. (His team was unable to observe FRB 200428 itself.)

“This is puzzling, but very interesting,” Zhang says.

Further compounding the puzzle is that some of the gamma-ray bursts not accompanied by FRBs were brighter than the one associated with FRB 200428.

Perhaps, Zhang says, these other flares actually did produce FRBs, but emitted them in narrow beams that didn’t happen to point our way. Or perhaps only “very special” types of gamma-ray-creating events produce FRBs. “We don’t have a very clear conclusion yet,” he says.

What we do know is that FRBs can indeed be created by magnetars. “Before, we had more than 15 different models.”

Meanwhile, astronomers are hoping to see more FRBs like FRB 200428 in our own galaxy. “We’ll see how special, or not special, it is,” Zhang says.

How long we have to wait for that, Bochenek adds, is unknown. “We don’t know how lucky we got,” he says. “This could a once-in-five-year event, or it could be every few months.”

Bochenek’s, Michilli’s, and Zhang’s teams’ studies all appear in today’s issue of Nature.

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