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Artistic impression of an exploding magnetar, showing a complex structure of the magnetic field and the emission of rays, here imagined as following an episode of rupture of the crust. Credit: McGill University graphic design team
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 emit over 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 throughout the universe, but their sources were too far away to be clearly distinguished. It was a mystery, therefore, about which astrophysical objects could produce such short but brilliant radio flares.
Now astronomers of WITH, McGill University, the University of British Columbia, the University of Toronto, the Perimeter Institute for Theoretical Physics and elsewhere report seeing fast radio bursts in our galaxy for the first time. Radio pulses are the closest FRBs detected to date, and their proximity has allowed the team to pinpoint their source.
It appears that the radio pulses observed were produced by a magnetar, a type of neutron star with an extremely powerful magnetic field. Physicists have speculated that magnetars could produce FRBs. This is the first time that scientists have had direct observational evidence that magnetars are indeed sources of fast radio bursts.
“There is this great mystery about what these great bursts of energy would produce, which until now we have seen come from half the universe,” says Kiyoshi Masui, assistant professor of physics at MIT, who led the analysis of the team of FRB brightness. “This is the first time we have been able to link one of these exotic fast radio bursts to a single astrophysical object.”
The researchers published their findings in the journal Nature on November 4, 2020, 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.
Bursts of staccato
Astronomers detected signals from an FRB using the CHIME radio telescope at the Dominion Radio Astrophysical Observatory in British Columbia. CHIME consists of four large reflectors, each as long as a football field and similar to skateboard halfpipes, which focus incoming radio waves on over a thousand antennas. Together, the antennas continuously monitor the bands of the sky for incoming radio waves – energy emitted within the radio band of the electromagnetic spectrum.
Towards the end of April 2020, astronomers detected bursts of activity in the X-ray band of the spectrum from a magnetar in the Milky Way, towards the center of the galaxy and about 30,000 light years from Earth. The magnetar is part of a handful of known magnetars in our galaxy and until April it was, as Masui describes it, an “ordinary” magnetar. Astronomers have labeled it SGR 1935 + 2154, due to its coordinates in the sky.
“There was a certain buzz in the astronomy community about this magnetar that had become active in X-rays, and it was mentioned in our collaboration that we should be keeping an eye out for more of this magnetar,” Masui says.
Sure enough, the team found that shortly after the magnetar exploded in the X-ray band, CHIME detected two sharp, sharp peaks in the radio band, within milliseconds of each other, signaling a rapid 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 emitted X-rays at the same time.
“If it came from any other object close to the magnetar, it would be a great coincidence,” Masui says.
Masui then led the effort to measure the magnetar’s brightness as it generated the radio bursts, a difficult task, as the explosions were detected on the outskirts of CHIME, where the telescope is less sensitive and its instrumentation is more difficult to interpret. Then the team used calibration data from other astrophysical sources to estimate the brightness of the magnetar.
In the end, they calculated that the magnetar, in the split second that the FRB flashed, was 3000 times brighter than any other magnetar radio signal that has yet been observed.
“Measuring its brightness was what determined it was not a normal pulse,” says Masui. “This is a fast radio flash that occurs in our galaxy, which is thousands of times brighter than any other pulse we’ve ever seen.”
“Eyes open”
Now that magnetars have been shown to produce fast radio bursts, the question remains: how? Although proposals abound, scientists are unsure exactly how FRBs are generated in the universe and, in particular, how magnetars could 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 found in galaxies.
But physicists suspect that magnetars can generate radio waves through an entirely different process, in which electrons, instead of randomly interacting with a magnetic field, do so en masse. This “coherent” process would be similar to the way 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 going through space, are causing this radio emission that we see,” Masui says. “The mechanics of how this happens astrophysically, in magnetars or pulsars, is not well understood.”
Since the team discovered the radio exploding magnetar, other groups have trained several telescopes on the source and reported that the magnetar emitted successive radio bursts, although not as intense as the initial FRB.
“He’s done some interesting things and we’re trying to piece together the meaning of it all,” says Masui. “We have our eyes open for other magnetars, but the important thing now is to study this single source and dig deeper to see what it tells us about how FRBs are made.”
Read Astronomers Detect Short, Intense Radio Burst from Relatively Near in the Milky Way for more on this study.
Reference: “A bright millisecond-duration radio burst from a Galactic magnetar” by the CHIME / FRB Collaboration, 4 November 2020, Nature.
DOI: 10.1038 / s41586-020-2863-y
This research was funded, in part by the Canada Foundation for Innovation and other supporting institutions.
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