9.11.2020

The fast radio bursts are likely generated by a magnetar, the most magnetic type of star in the universe.

Fast radio bursts are extremely bright flashes of energy that last for a fraction of a second, during which they can blast out more than 100 million times more power than the sun.

Since they were first detected in 2007, astronomers have observed traces of fast radio bursts, or FRBs, scattered across the universe, but their sources have been too far away to clearly make out. It has been a mystery, then, as to what astrophysical objects could possibly produce such brief though brilliant radio flares.

Now astronomers at MIT, McGill University, the University of British Columbia, the University of Toronto, the Perimeter Institute for Theoretical Physics, and elsewhere report that they have observed fast radio bursts in our own galaxy, for the first time. The radio pulses are the closest FRBs detected to date, and their proximity has allowed the team to pinpoint their source.

It appears that the observed radio pulses were produced by a magnetar — a type of neutron star with a hugely powerful magnetic field. Physicists have hypothesized that magnetars might produce FRBs. This is the first time scientists have direct observational proof that magnetars are indeed sources of fast radio bursts.

“There’s this great mystery as to what would produce these great outbursts of energy, which until now we’ve seen coming from halfway across the universe,” says Kiyoshi Masui, assistant professor of physics at MIT, who led the team’s analysis of the FRB’s brightness. “This is the first time we’ve been able to tie one of these exotic fast radio bursts to a single astrophysical object.”

The researchers have published their results today in the journal Nature. The authors are all members of the Canadian Hydrogen Intensity Mapping Experiment (CHIME) Collaboration, a team of over 50 scientists led by McGill University, the University of British Columbia, the University of Toronto, the Perimeter Institute for Theoretical Physics, and the National Research Council of Canada.

Staccato bursts

The astronomers picked up signals of an FRB using the CHIME radio telescope at the Dominion Radio Astrophysical Observatory in British Columbia. CHIME is made up of four large reflectors, each as long as a football field and resembling skateboarding halfpipes, which focus incoming radio waves onto over a thousand antennas. Together, the antennas continuously monitor swaths of the sky for incoming radio waves — energy emitted within the radio band of the electromagnetic spectrum.

Toward the end of April 2020, astronomers picked up some bursts of activity, in the X-ray band of the spectrum, from a magnetar in the Milky Way, toward the galaxy’s center and about 30,000 light years from Earth. The magnetar is among a handful of known magnetars in our galaxy, and until April, it was, as Masui describes, a “run-of-the-mill” magnetar. Astronomers labeled it SGR 1935+2154, for its coordinates in the sky.

“There was some buzz in the astronomy community about this magnetar that had become active in the X-ray, and it had been mentioned within our collaboration that we should keep an eye out for something more from this magnetar,” Masui says.

Sure enough, the team found that, shortly after the magnetar burst in the X-ray band, CHIME picked up two sharp staccato peaks in the radio band, within several milliseconds of each other, signaling a fast radio burst. The researchers were able to track the radio bursts to a point in the sky that was within a fraction of a degree of SGR 1935+2154 — the same magnetar that was blasting out X-rays around the same time.

“If it was coming from any other object close to the magnetar, it would be a very big coincidence,” says Masui.

Masui then led the effort to measure the brightness of the magnetar as it was generating the radio bursts — a tricky task, as the bursts were detected at CHIME’s periphery, where the telescope is less sensitive, and its instrumentation is trickier to interpret. So the team used calibration data from other astrophysical sources to estimate the magnetar’s brightness.

In the end, they calculated that the magnetar, in the fraction of a second that the FRB flashed, was 3,000 times brighter than any other magnetar radio signal that has yet been observed.   

“Measuring its brightness was really what established this is not a normal pulse,” Masui says. “This is a fast radio burst happening in our own galaxy, that is thousands of times brighter than any other pulse we’ve ever seen.”

“Eyes open”

Now that it’s been shown that magnetars can produce fast radio bursts, the question remains: how? While proposals abound, scientists are unsure of exactly how FRBs are generated in the universe, and specifically, how magnetars might produce them.

Most radio emissions in the universe are produced through a process known as synchrotron radiation, in which a gas of randomly moving high-energy electrons interacts with magnetic fields, in a way that emits energy at radio frequencies. Radio waves are often generated in this way by supermassive black holes, supernova remnants, and hot gas sitting in galaxies.

But physicists suspect that magnetars may generate radio waves through an entirely different process, where electrons, instead of interacting randomly with a magnetic field, are doing so en masse. This “coherent” process would be similar to the way in which we generate radio waves on Earth, by directing electrons through a wire, in the same direction.

“We think some sort of process like that, some coherent currents running through space, is causing this radio emission that we see,” Masui says. “The mechanics of how that happens astrophysically, in magnetars or pulsars, is not well-understood.”

Since the team discovered the radio-bursting magnetar, other groups have trained different telescopes on the source, and have reported that the magnetar has emitted subsequent radio bursts, though not nearly as intense as the initial FRB. 

“It’s been doing interesting things, and we’re trying to piece together what it all means,” Masui says. “We’ve got our eyes open for other magnetars, but the big thing now is to study this one source and really drill down to see what it tells us about how FRBs are made.”

This research was funded, in part by the Canada Foundation for Innovation, and other supporting institutions.

Quelle: MIT Massachusetts Institute of Technology