When a magnetar within the Milky Way galaxy belched out a flare of colossally powerful radio waves in 2020, scientists finally had concrete evidence to pin down an origin for fast radio bursts.
A mind-blowing new study has now narrowed down the mechanism. By studying the twinkling light of a fast radio burst detected in 2022, a team of astronomers has traced its source to the powerful magnetic field around a magnetar, in a galaxy 200 million light-years away.
It’s the first conclusive evidence that fast radio bursts can emerge from the magnetospheres of magnetars.
“In these environments of neutron stars, the magnetic fields are really at the limits of what the Universe can produce,” says astrophysicist Kenzie Nimmo of the Massachusetts Institute of Technology (MIT).
“There’s been a lot of debate about whether this bright radio emission could even escape from that extreme plasma.”
Fast radio bursts (FRBs) have puzzled scientists since they were first discovered in 2007. They are, as the name suggests, extremely brief bursts of radio emission, lasting just milliseconds. They’re also extremely powerful, sometimes releasing more energy than 500 million Suns in that brief blink of time.
FRBs are hard to study because most of the time, they burst only once. This makes them impossible to predict, and tricky – but not impossible – to trace back to a source. A number of one-off FRBs have been traced to galaxies across millions to billions of light-years of space-time.
Astronomers can also examine the properties of the radio light, such as its polarization, to figure out what type of environment it traveled through on its way to Earth. What kinds of stars might emit FRBs is still largely a mystery, but a growing body of evidence increasingly implicates magnetars.
Magnetars are particularly unusual neutron stars, which themselves are the extremely dense core remnants left over after a massive star goes supernova. But magnetars have much more powerful external magnetic fields than ordinary neutron stars – around 1,000 times stronger. They’re the most powerful magnetic fields in the Universe.
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“Around these highly magnetic neutron stars, also known as magnetars, atoms can’t exist – they would just get torn apart by the magnetic fields,” says physicist Kiyoshi Masui of MIT.
“The exciting thing here is, we find that the energy stored in those magnetic fields, close to the source, is twisting and reconfiguring such that it can be released as radio waves that we can see halfway across the Universe.”
To trace the origin of an FRB, Nimmo and her colleagues studied a property known as scintillation in an event known as FRB 20221022A, first detected in 2022 and subsequently traced to a galaxy 200 million light-years away. Scintillation is what makes stars twinkle – the distortion of the path of light as it travels through gas in space. The longer the distance traveled, the stronger the twinkling.
FRB 20221022A is pretty bog standard, as far as FRBs go. It was moderately long, around 2 milliseconds, and moderately powerful. This makes it an excellent case study for trying to understand the properties of other FRBs, too.
A companion paper studying the polarization of the light from FRB 20221022A – the degree to which the orientation of its waves is twisted – found an S-shaped angle swing consistent with a rotating object, a first for an FRB. This suggested that the signal originated from very close to the rotating object.
Nimmo and colleagues figured out that, if they could determine the degree of scintillation in FRB 20221022A, they could calculate the size of the region it originated from. The light from the FRB showed strong scintillation, leading the researchers to the gas region that distorted the signal. By using that gas region as a lens, they narrowed down the source of the FRB to within 10,000 kilometers (6,213 miles) of its magnetar source.
“Zooming in to a 10,000-kilometer region, from a distance of 200 million light years, is like being able to measure the width of a DNA helix, which is about 2 nanometers wide, on the surface of the Moon,” Masui says. “There’s an amazing range of scales involved.”
It’s the first conclusive evidence that extragalactic FRBs can originate from within the magnetosphere of highly magnetized neutron stars. But it’s more than that. The techniques used by the team show that scintillation may be a powerful probe for other FRBs, so astronomers can try to understand how diverse they might be – and whether other kinds of stars might also belch out the powerful eruptions.
“These bursts are always happening,” Masui says. “There may be a lot of diversity in how and where they occur, and this scintillation technique will be really useful in helping to disentangle the various physics that drive these bursts.”
The research has been published in Nature.
