There are strange radio signals pinging us here on Earth. It’s possible to feel them thousands of times a day if astronomers know where to find them.
These are most likely not attempts by aliens to contact us. Astronomers call them Fast Radio Bursts (FRBs), and they’re among the most puzzling space mysteries of our time. We’re starting to get an idea of where they might be coming from, but we’re not sure what exactly is causing them.
Astronomers are working on it. Researchers from Nanjing University and Hong Kong University modeled what might shape one of them and examined the rapidly repeating burst called FRB 20201124A in a paper published in Nature Communications on Sept. 21.
Fast radio bursts are short: most last a second or two or less. They are outbursts: when they occur, they are considered to be as energetic as our sun. However, by the time the signals reach us, they’re generally much weaker than our terrestrial radio waves – which partly explains why it’s taken them so long to find them.
Astronomers have observed these small blips in their radio telescopes for more than a decade. In 2007, astronomers combing through six-year-old data found a short, brief pulse of unknown origin. So far it was the first of hundreds.
signals from the unknown
What causes FRBs remains unclear, if there is any single explanation at all. Astrophysicists have suggested connections to black holes, neutron stars, gamma-ray bursts, supernovae, and all sorts of other distant (yes, even extraterrestrial) phenomena.
A popular culprit is a magnetar: a specific type of high-energy neutron star with an extremely strong magnetic field, up to a trillion times stronger than Earth’s. In 2020, astronomers discovered an FRB emanating from a magnetar in our own galaxy.
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Even then, what exactly causes a magnetar to generate an FRB is unknown. Some astronomers suspect that this has to do with how magnetars spin, which could produce the predictable beats of certain FRBs — similar to the clockwork-precise timings of a spinning pulsar. Astronomers call this attribute “periodicity”. But in many cases there is no evidence for this. (Another theory is that some FRBs come from disks of gas and dust accumulating around black holes.)
What complicates matters further is that each of these hundreds of FRBs is a different animal. Some flash once, never to be seen again. Some blink a few times. Some stay silent for days, then randomly light up for a short time and then go silent again. And some blink dozens of hundred times in quick succession. FRB 20201124A clearly belongs to the latter category.
Hunt for FRB 20201124A
Astronomers first saw it in November 2020 (hence the numbering of its name). They caught a glimpse of its chime with, well, CHIME – a radio telescope in British Columbia now tasked with searching for FRBs’ fingerprints. Every day, CHIME sweeps across the sky, pausing in one spot for a few minutes. During one of these pauses, the oscilloscope found FRB 20201124A.
At first it appeared to be just another FRB. “We didn’t announce it right away,” says Adam Lanman, a postdoctoral astrophysicist at McGill University who helped discover CHIME. That would change soon.
In April 2021, CHIME spotted FRB 20201124A metaphorically flashing and emitting repetitive pulses. CHIME astronomers alerted the world’s astronomy community. “After that, a number of other observatories started seeing a lot of events from him,” Lanman says.
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One such observatory was FAST: the world’s largest radio telescope, nestled in the mountains of southwest China’s Guizhou province. In another article published in Nature on the same day, scientists using FAST reported seeing nearly 2,000 more explosions from FRB 20201124A before the source went silent again.
“This large sample can help us shed light on the origins of FRBs,” says Wang Fayin, an astrophysicist at Nanjing University.
Repetitive FRBs aren’t new, but FAST’s observations saw a series of unique fingerprints in the radio waves that indicated something was playing with them. “There are some unique properties of FRB 20201124A that motivate us to create a model for it,” says Wang.
An exemplary star system
Wang and his colleagues attempted a model. Theirs suggest that FRB 20201124A came from a magnetar — but not from a magnetar alone. When radio waves erupt from the magnetar, they pass the edge of the star that the magnetar orbits. It’s a special type of star called the Be star, a very bright star cloaked in a disk of plasma and gas. The radio waves from an FRB would pass through this disk and explain its unique properties.
“It’s all completely speculative, but none of it is impossible,” says Jonathan Katz, an astrophysicist at Washington University in St. Louis who was not an author.
“I haven’t seen any other paper that goes into as much detail as this one,” says Lanman, who was also not an author.
But this model doesn’t fit the FAST data perfectly – there are some variations that it doesn’t fully explain. “Whatever is going on, at its core, it may have its model, but there’s a lot more going on,” says Katz.
Modeling FRBs in this way is not new. Astronomers have often thought that repetitive FRBs are due to a neutron star or black hole orbiting another star. On the other hand, it is not yet clear how exactly FRB 20201124A repeats itself. Katz says outside groups have not yet been able to search the FAST data for evidence of periodicity.
However, when astronomers are looking for a magnetar orbiting another star, they also know where to find it. The same observations that produced the model helped narrow the source of FRB 20201124A to a specific galaxy, which may later help astronomers find it. They could do that by looking at other wavelengths: X-rays, for example, or gamma rays.
Astronomers have previously attempted to probe this galaxy with X-rays. But the model could help them narrow down their searches, and that’s what Lanman recommends after this work: “Of course, further searches for X-ray counterparts are warranted in the future,” says Lanman.