Mysterious radio bursts from outer space first discovered in 2007, last only a millisecond but can carry an enormous amount of energy—enough to briefly outshine entire galaxies.
Since that first fast radio burst, or FRB, astronomers have detected thousands more, whose locations range from within our own galaxy to as far as 8 billion light-years away—yet, exactly how these brief and brilliant explosions were launched had remained a highly-contested unknown.
Now, astronomers at the Massachusetts Institute of Technology (MIT) have pinned down the origin of at least one of these cosmic radio flares using a novel technique that could do the same for other FRBs.
In their new study, published this week in the journal Nature, the team focused on a previously discovered fast radio burst that was detected from a galaxy about 200 million light-years away.
They zeroed in to determine the precise location of the radio signal by analyzing its “scintillation,” which is similar to how stars twinkle in the night sky.
The scientists studied changes in the FRB’s brightness and determined that the burst must have originated from the immediate vicinity of its source, rather than much further out, as some models have predicted.
The fleeting fireworks known as FRB 20221022A exploded from a region that is extremely close to a rotating neutron star, up to 10,000 kilometers away—less than the distance between New York and Singapore.
At such close range, the burst likely emerged from the neutron star’s magnetosphere—a highly magnetic region immediately surrounding the extremely compacted star.
The team’s findings provide the first conclusive evidence that a FRB can originate from the magnetosphere immediately surrounding an ultracompact object, such as a neutron star or possibly a black hole.
“In these environments of neutron stars, the magnetic fields are really at the limits of what the universe can produce,” said the study’s lead author Kenzie Nimmo, a postdoc in MIT’s Institute for Astrophysics and Space Research. “There’s been a lot of debate about whether this bright radio emission could even escape from that extreme plasma.”
“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 Kiyoshi Masui, associate professor of physics at 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.”
Detections of FRBs have soared since 2020, thanks to the Canadian Hydrogen Intensity Mapping Experiment (CHIME).
The radio telescope array comprises four large, stationary receivers, each shaped like a half-pipe, that are tuned to detect radio emissions within a range that is highly sensitive to fast radio bursts.
The exact physics driving the FRBs have remained unclear. Some models predict that the should come from the turbulent magnetosphere immediately surrounding a compact object, while others predict that the bursts should originate much further out, as part of a shockwave that propagates away from the central object.
To determine where FRBs arise, the MIT team considered scintillation, the effect that occurs when light from a small bright source such as a star, filters through some medium, such as a galaxy’s dense gas.
As the starlight filters through the gas, it bends in ways that make it appear, to a distant observer, as if the star is twinkling. The smaller or the farther away an object is, the more it twinkles.
The light from larger or closer objects, such as planets in our own solar system, experience less bending, and therefore do not appear to twinkle.
The team reasoned that if they could estimate the degree to which an FRB scintillates, they might determine the relative size of the region from where the FRB originated. The smaller the region, the closer in the burst would be to its source, and the more likely it is to have come from a magnetically turbulent environment. The larger the region, the farther the burst would be, giving support to the idea that FRBs stem from far-out shockwaves.
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Twinkle, twinkle neutron star
To test their idea, the researchers looked to FRB 20221022A, a signal that lasts about two-thousandths of one second, which is average for FRBs, in terms of its brightness.
Collaborators at McGill University in Canada found that it exhibited one standout property: The light from the burst was highly polarized, with the angle of polarization tracing a smooth S-shaped curve.
The pattern is interpreted as evidence that the FRB emission site is rotating—a characteristic previously observed in pulsars, which are highly magnetized, rotating neutron stars.
This is a first for FRBs, suggesting that the signal may have arisen from the close-in vicinity of a neutron star.
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The MIT team realized that if FRB 20221022A originated from close to a neutron star, they should be able to prove this, using scintillation.
Dr. Nimmo and her colleagues analysed data from CHIME and observed steep variations in brightness that signalled scintillation — in other words, the FRB was twinkling.
They confirmed that there is gas somewhere between the telescope and FRB that is bending and filtering the radio waves.
The team then determined where the gas could be located, confirming that gas within the FRB’s host galaxy was responsible for some of the scintillation observed. The gas acted as a “natural lens” – allowing the researchers to zoom in on the FRB site and determine that the burst originated from an extremely small region, estimated to be about 10,000 kms wide.
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“This means that the FRB is probably within hundreds of thousands of kilometers from the source,” said Nimmo. “That’s very close. For comparison, we would expect the signal would be more than tens of millions of kilometers away if it originated from a shockwave, and we would see no scintillation at all.”
“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,” Dr. Masui said.
The findings prove for the first time that FRBs can originate from very close to a neutron star, in highly chaotic magnetic environments.
“These bursts are always happening, and CHIME detects several a day,” Masui added. “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.”
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