There are strange radio signals sending us here to Earth. It’s possible to smell them thousands of times every day, if astronomers know where to find them.
These are probably not attempts by extraterrestrials to contact us. Astronomers call them fast radio bursts (FRBs), and they’re among today’s trickiest space mysteries. We’re starting to get an idea of where they might be coming from, but we don’t know exactly what’s causing them.
Astronomers are working on it. Researchers from Nanjing University and the University of Hong Kong have modeled what might shape one of them, in a paper published in Nature Communications on September 21, studying the fast repeating burst named FRB 20201124A.
Fast radio bursts are brief: most last a second or two or less. They are shards: when created, they are believed to be as energetic as our sun. That said, by the time the signals reach us, they’re usually much weaker than our terrestrial radio waves, which is part of why they’ve taken so long to find them.
Astronomers have observed these tiny dots 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. It was the first of hundreds, so far.
Signals from the unknown
What causes FRBs, if they have a single explanation, remains unclear. Astrophysicists have suggested links to black holes, neutron stars, gamma-ray bursts, supernovae, and all sorts of other distant phenomena (yes, even aliens).
A popular culprit is a magnetar: a certain type of high-energy neutron star with an extremely strong magnetic field, up to a trillion times the strength of Earth. In 2020, astronomers Point an FRB emanating from a magnetar in our own galaxy.
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Even then, it is unclear exactly what causes a magnetar to generate an FRB. Some astronomers suspect it has to do with the way magnetars spin, which could create the predictable beats of some FRBs, much like the precise timings of a spinning clock. pulsar. Astronomers call this attribute “periodicity.” Yet in many cases there is no evidence of this. (Another theory is that some FRBs come from disks of gas and dust that accumulate around black holes.)
To complicate matters, each of those hundreds of FRBs is a different beast. Some blink once, never to be seen again. Some flash multiple times. Some go silent for days, then turn on randomly for a short time, then go silent again. And some flash tens of hundreds of times in quick succession. FRB 20201124A definitely belongs to the latter category.
Flush for FRB 20201124A
Astronomers first saw it in November 2020 (hence the numbering of its name). They caught sight of her chiming with, well, CARILLON– a radio telescope in British Columbia that is now responsible for looking for fingerprints of FRBs. Every day, CHIME scans the sky, stopping in one spot for a few minutes at a time. It was during one of these pauses that the oscilloscope found FRB 20201124A.
At first it looked like another FRB. “We didn’t announce it right away,” says Adam Lanman, a postdoctoral astrophysicist at McGill University who was involved in the discovery of CHIME. That would soon change.
In April 2021, CHIME spotted FRB 20201124A metaphorically lighting up, sending out repeated pulses. CHIME astronomers have alerted the global astronomy community. “After that, a bunch of other observatories started seeing a lot of events,” Lanman says.
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One such observatory was FAST: the world’s largest radio telescope, nestled in the mountains of Guizhou Province in southwest China. In another newspaper published in Nature the same day, scientists using FAST reported seeing nearly 2,000 more explosions of FRB 20201124A before the source went silent again.
“This large sample may help shed some light on the origins of FRBs,” says Wang Fayinastrophysicist at Nanjing University.
Repeating FRBs aren’t new, but FAST observations saw a number of unique fingerprints in radio waves that suggested something was playing with them. “FRB 20201124A has some unique features, which motivates us to create a model for it,” Wang said.
A model star system
Wang and his colleagues tried their hand at a model. Theirs suggests that FRB 20201124A is from a magnetar, but not from a magnetar alone. When radio waves shoot out from the magnetar, they pass through the skirt of the star around which the magnetar orbits. It is a special type of star called a Be star, a very bright star surrounded by a disk of plasma and gas. Radio waves from an FRB would pass through this disc, explaining their unique characters.
“This is all completely speculative, but none of this is impossible,” says Jonathan Katzastrophysicist at Washington University in St. Louis, who was not an author.
“I haven’t seen any other articles as detailed as this one,” says Lanman, who was also not an author.
But this model doesn’t fit the FAST data perfectly – there’s quite a bit of variation that it doesn’t fully explain. “Whatever happens might have their role model at heart, but there’s a lot more going on than that,” Katz says.
Modeling FRBs in this way is nothing new. Astronomers have often thought that the repeating FRBs were due to a neutron star or a black hole orbiting another star. On the other hand, it is not yet known exactly how FRB 20201124A repeats itself. Katz says outside groups have not yet been able to sift through FAST data to find evidence of periodicity.
Yet if it’s a magnetar orbiting another star that astronomers are looking for, they also know where to find it. The same observations that produced the model helped narrow the source of FRB 20201124A to a particular galaxy, which may help astronomers find it later. They could do this by searching in other wavelengths: X-rays, for example, or gamma rays.
Astronomers have tried scanning this galaxy with X-rays before. But the model could help them refine their search attempts, and that’s what Lanman recommends after this work: “Certainly, further searches for counterparts to the X-rays in the future” are in order, says Lanman.