For years, astronomers have puzzled over the origins of phenomena known as fast radio bursts — cosmic emissions that last only a fraction of a second but blast out more than 100 million times more power than our sun. Some even wondered whether the bursts, known as FRBs, might serve as signals from extraterrestrial civilizations.
Now they’ve tracked down the source of the first fast radio burst detected in our own Milky Way galaxy — and it’s not aliens. Instead, it’s a magnetar, a type of neutron star with a powerful magnetic field.
Scientists have long suspected that fast radio bursts had something to do with magnetars. But the newly reported case, described in three studies published today by the journal Nature, serves as the astronomical equivalent of a smoking gun.
“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,” Kiyoshi Masui, a physicist at the Massachusetts Institute of Technology, said today in a news release. “This is the first time we’ve been able to tie one of these exotic fast radio bursts to a single astrophysical object.”
Masui is part of the team that picked up the first clues to the source, a magnetar 30,000 light-years from Earth that’s known as SGR 1935+2154. The team includes researchers from MIT, the University of British Columbia, McGill University, the University of Toronto and the Perimeter Institute for Theoretical Physics.
They made use of a radio telescope array called the Canadian Hydrogen Intensity Mapping Experiment, or CHIME, which began science operations in 2018 at the Dominion Radio Astrophysical Observatory in British Columbia’s Okanagan Valley.
In April, astronomers detected bursts of X-ray and gamma-ray activity from SGR 1935 — which led the CHIME team to turn their attention to that part of the sky, around the center of the Milky Way. Shortly after an X-ray burst on April 28, CHIME registered two sharp peaks in radio emissions, within a few milliseconds of each other.
That fit the pattern for a fast radio burst, emanating from a point in the vicinity of SGR 1935. “If it was coming from any other object close to the magnetar, it would be a very big coincidence,” Masui said.
The source was near the edge of CHIME’s field of view, which made it difficult to determine the radio burst’s brightness. So the team put out the word for other astronomers to check their records.
By a stroke of luck, another radio astronomy project — known as the Survey for Transient Astronomical Radio Emission 2, or STARE2 — had a wide-field view of the same blast.
“When I saw the data, I was basically paralyzed,” Caltech graduate student Christopher Bochenek said in a news release. “At the radio frequencies we observe with STARE2, the signal was much stronger than what CHIME reported. We had caught the FRB head-on.”
STARE2 isn’t your typical radio telescope array: The heart of the Caltech-led, NASA-funded project is a handmade radio receiver that’s about the size of a large bucket. “It’s a piece of 6-inch metal pipe with two literal cake pans around it,” Bochenek told The Associated Press.
Three of the receivers are placed at widely separated locations in California and Utah, which makes it possible to triangulate on the source of cosmic radio emissions. They’re not as sensitive as the more traditional big-dish telescopes, but they can take in the whole sky.
.@Caltech's Chris Bochenek, who played a role in tracking down a Milky Way fast radio burst, stands next to a STARE2 receiver. https://t.co/sziWf0iqlF (Photo composite: R. Hunt / @caltechipac) pic.twitter.com/gBrdj1RFss
— Alan Boyle (@b0yle) November 5, 2020
The readings from STARE2, combined with data from other instruments, suggested that the April 28 burst was 3,000 times brighter than any previously observed magnetar radio signal.
Among the other instruments participating in the observational campaign was China’s Five-Hundred-Meter Aperture Spherical Radio Telescope, also known as FAST. Astronomers on the FAST team missed out on detecting FRB 200428, but they kept an eye on SGR 1935 as it emitted a series of 29 gamma-ray bursts. None of those bursts coincided with a blast of high-energy radio waves.
“The weak correlation could be explained by special geometry and/or limited bandwidth of FRBs,” study co-author Zhang Bing of the University of Nevada at Las Vegas said in a news release. “The observations of SGR J1935 start to reveal the magnetar origin of FRBs, although other possibilities still exist.”
Astrophysicists haven’t yet figured out the mechanism for producing fast radio bursts, but one hypothesis is that they can occur when a magnetar throws off a flare of charged particles that interact with debris surrounding the star. The resulting shock wave could set electrons gyrating wildly, throwing off radio waves as well as X-rays.
To unravel that part of the mystery, the CHIME team and other astronomers are keeping a close watch on SGR 1935.
“We’ve got our eyes open for other magnetars,” Masui said, “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.”
The CHIME/FRB Collaboration’s study, described in a Nature paper titled “A Bright Millisecond-Duration Radio Burst From a Galactic Magnetar,” was funded by the Canada Foundation for Innovation and other supporting institutions. The second Nature study, “A Fast Radio Burst Associated With a Galactic Magnetar” counts Bochenek as well as V. Ravi, K.V. Belov, G. Hallinan, J. Kocz, S.R. Kulkarni and D.L. McKenna among its authors. Zhang is among 48 authors of the third Nature paper, titled “No Pulsed Radio Emission During a Bursting Phase of a Galactic Magnetar.”