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In 2007, examining a mine of radio telescope data, Duncan Lorimer, an astrophysicist at West Virginia University, spotted something unusual. The data obtained six years earlier showed a brief, forceful burst lasting no more than 5 milliseconds. Others had seen the blip and looked beyond, but Lorimer and his team calculated that it was an entirely new phenomenon: a signal from somewhere far from the Milky Way.
The team had no idea what caused it, but they published the results in Science. The mysterious signal became known as a “fast radio burst” or FRB. In the 13 years since Lorimer’s discovery, dozens of FRBs have been discovered outside the Milky Way – some repeated and others ephemeral, single chirps. Astrophysicists have been able to pinpoint their galaxies of origin, but have struggled to identify the cosmic culprit, advancing all sorts of theories, from exotic physics to alien civilizations.
On Wednesday, a trio of studies in the journal Nature describes the source of the first FRB discovered within the Milky Way, revealing the mechanism behind at least some of the highly energetic radio bursts.
The recently described burst, dubbed FRB 200428, was discovered and located after pinging radio antennas in the United States and Canada on April 28, 2020. A hasty hunt followed, with teams of researchers from around the world focusing on the study of the FRB through the electromagnetic spectrum. It was quickly determined that FRB 200428 is the most energetic radio pulse ever detected in our galaxy.
In the series of new articles, astrophysicists outline their investigative work and groundbreaking observations from a handful of ground and space telescopes. By linking concordant observations together, the researchers fix FRB 200428 on one of the cosmos’s most unusual wonders: a magnetar, the hypermagnetic remnants of a dead supergiant star.
It is the first time that astrophysicists have been able to pinpoint a culprit of the intergalactic whodunit – but this is only the beginning. “There’s really a lot to learn in the future,” says Amanda Weltman, an astrophysicist at the University of Cape Town and author of a Nature paper accompanying the discovery.
“This is just the first exciting step.”
Under pressure
To understand where FRB 200428 begins, you need to understand where a star ends.
Stars many times larger than the sun are known to suffer a disordered death. After they have used up all their fuel, physics conspires against them; their immense size exerts unfathomable pressure on their core. Gravity forces the star to fold in on itself, causing an implosion that releases huge amounts of energy in an event known as a supernova.
The crumpled core of the star, born under extreme pressure, is left behind. Except it’s now very small, only about the size of a city and about 1 million times denser than Earth. This stellar zombie is known as a neutron star.
Some neutron stars have extreme magnetic fields, about 1,000 times stronger than typical neutron stars. They are a mysterious and intriguing class. Astronomers call them “magnetars” and are as curious as FRB, with only about thirty discoveries so far.
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One such magnetar in the Milky Way is officially known as SGR 1935 + 2154, which refers to its position in the sky. To keep things simple, let’s call it Mag-1. It was first discovered in 2014 and is about 30,000 light-years from Earth. On April 27, 2020, NASA’s Neil Gehrels Swift Observatory and Fermi Gamma-ray Space Telescope detected a spike in X-rays and gamma rays emanating from Mag-1.
The next day, two huge North American telescopes – the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Survey for Transient Astronomical Radio Emission 2 (STARE2) – detected an extremely energetic radio discharge coming from the same region of space: FRB 200428. The FRB and Mag-1 were in exactly the same galactic neighborhood. Or rather, they appeared to be in the same galactic home.
“These observations point to the magnetars as a smoking gun from an FRB,” says Lorimer, lead author of the 2007 discovery of the first radio explosion. Magnetars had previously been theorized as potential sources of FRBs, but the data provide direct evidence linking the two cosmic phenomena together.
However, just co-localizing the burst with the magnetar doesn’t explain everything.
“Magnetars occasionally produce bursts of bright X-ray emission,” says Adam Deller, an astrophysicist at Swinburne University in Melbourne, Australia, “but most magnetars have never been seen to emit any radio emissions.”
Do not stop me now
Mag-1’s association with FRB 200428 is just the beginning of a long-term investigation.
In cosmic yellow, astronomers have found a culprit, but they’re not exactly sure about the murder weapon.
By studying the FRB, the researchers were able to determine that it was highly energetic but paled in comparison to some previously discovered deep-space FRBs. “It was almost as bright as the faintest FRBs we detected,” says Marcus Lower, a doctor of astronomy. at Swinburne University studying neutron stars. This suggests that the magnetars could be responsible some FRB but not all – some seem too energetic to be produced the same way FRB 200428 was.
Another article in Nature on Wednesday sees researchers using China’s Five Hundred Meter Aperture Spherical Radio Telescope (FAST) to study Mag-1 during one of its X-ray bursts. The telescope detected no radio emission from the magnetar during its explosions. This means that such a vent alone is unlikely to be responsible for escaping highly energetic FRBs. “It’s clear that not all bursts of magnetar X-rays emit an accompanying radio blast,” says Deller.
Deller further notes that FRB 200428 exhibits similar characteristics to those seen in repeating FRB from outside the Milky Way.
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This is important because, at the moment, astronomers have observed two types of FRBs in other galaxies. There are those that come to life and disappear, and others that seem to repeat themselves on a regular basis. FRB 200428 looks like a repeater, but much weaker. Further observations by the CHIME telescope in October detected more radio bursts from the magnetar, although this work has not yet been published.
All in all, there is still some uncertainty. “We can’t say for sure whether magnetars are the source of all the FRBs observed to date,” Weltman notes.
Another question: how did Mag-1 generate the FRB? Two different mechanisms have been proposed.
One suggestion is that magnetars produce radio waves just as X-rays and gamma rays do in their magnetosphere, the huge region of extreme magnetic fields that surround the star. The other is a bit more complex. “The magnetar could live in a cloud of pending material from previous streams,” says Adelle Goodwin, an astrophysicist at Curtin University who was not affiliated with the study. This cloud of material, Goodwin notes, could then be hit by an X-ray or gamma-ray explosion, transferring energy into radio waves. Those waves then travel through the cosmos and make noise with Earth’s detectors such as an FRB.
It is not clear what mechanism led to FRB 200428 or if something more exotic could happen. Other researchers have suggested that FRBs could also be caused by asteroids hitting a magnetar, for example. But one thing now seems certain: they are not alien civilizations trying to contact us. Sorry.
Radio ga ga
There is still a lot of work to be done to unravel the mystery of fast radio bursts.
For Deller, the hunt continues. Part of his work focuses on where is it FRBs originate. He says his team has yet to collect more data, but there’s a chance that FRB’s repeat may be different types galaxies from those FRBs that do not repeat. Weltman notes that the search for other signals will also intensify, with astronomers looking for electromagnetic radiation and neutrinos generated by any FRBs produced by the magnetar.
The investigation will ultimately change the way we see the universe. Duncan Lorimer notes that if FRBs can be permanently linked to neutron stars, it would provide a way to take a census of those extreme cosmic entities. Current methods are unable to identify neutron star types with great specificity, but FRBs could change that. And FRBs are already changing the way we see things. A study published in Nature earlier this year used FRB to solve a decades-old problem about the universe’s “missing matter”.
Lorimer says many of the predictions his team made after discovering the first FRB in 2007 “have been realized in some way” and he always hoped that FRBs could enter the mainstream. As the mysteries deepen, they have exceeded his expectations. They have become one of the most puzzling but intriguing phenomena in astrophysics.
“It continues to be a fascinating adventure,” he says.
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