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For more than a decade, scientists have spotted weird radio signals in space that flash for a fraction of a second with an intense brightness that hints at mysterious and energetic sources. Dozens of these fast radio bursts, or FRBs, have been discovered—including one-off bursts and FRBs that emit multiple flashes, sometimes in clockwork patterns—yet their origins remain unknown.
Now, scientists led by Fayin Wang, an astronomer at Nanjing University in China, think they may have identified the likely source of one of the most enigmatic of all FRBs, which is known as FRB 20201124A. Since it was discovered in November 2020, this repeating burst has been seen going through super-charged periods of activity marked by many high-energy flashes, which have helped scientists trace its location to a galaxy some 1.3 billion miles from Earth.
Wang and his colleagues examined new images of the FRB obtained by China’s Five-hundred-meter Aperture Spherical radio Telescope (FAST), which is the largest single-dish radio telescope on Earth. The observations revealed patterns that are similar to a system in our own galaxy that contains two extreme objects: a magnetar, which is a highly magnetic type of dense dead star, and a Be star, which is an extremely hot and rapidly spinning type of star.
The researchers concluded that the same type of system “can naturally explain the mysterious features of FRB 20201124A,” according to a study published on Wednesday in Nature Communications.
“We propose that FRB 20201124A is produced by a magnetar residing in a binary system with a Be star companion with a disk,” Wang said in an email to Motherboard. “The interaction between radio bursts and the disk of Be star can naturally explain the observed unusual characteristics of FRB 20201124A.”
FRB 20201124A has been a head-scratcher to astronomers because its light is imprinted with features that are not seen in other FRBs. For instance, it is the first FRB to display variations in a measurement called Faraday rotation. This rotation describes the twists in the direction of polarization at different radio frequencies, which creates a pattern in the observations that can reveal insights about the environment around an FRB. While Faraday rotation has been spotted in other bursts, Wang noted that the measurement varies over time in FRB 20201124A.
“It is the first FRB showing Faraday rotation measure (RM) variations,” he said. “It has a short-time variation of the RM during the first 36 days of FAST observations, followed by a constant RM during the later 18 days.”
This shifting rotation measure implies that the magnetic field of the FRB source reverses along our line of sight, creating the distinctive pattern. This particular feature, among others, “can put strict constraints on the local environment of FRB 20201124A,” Wang noted.
With that in mind, the new study proposes that the FRB’s periods of rapid flashes are produced by energetic interactions between the magnetar and the disk of the Be star during “periastron,” which is the point when these two objects are closest together in their orbit. During this close approach, radio waves emitted by the roiling magnetar ripple through the disk of the Be star, producing the strange signatures seen in the FRB.
These new insights stem from FAST’s exceptional observations of FRB 20201124A, which are described in a companion study published at the same time in Nature. The telescope detected a whopping 1,863 independent bursts from April 1 to June 11, 2021, which “provide evidence for a complicated, dynamically evolving, magnetized immediate environment” around this FRB, according to the other study.
Wang and his colleagues hope that future observations might reveal even more details about this system, including the time it takes for the objects to orbit each other. The researchers also plan to apply their findings to another similar repeating burst called FRB 20190520B.
“Our model predicts the RM evolution would be quasi-periodic,” Wang said. “If a large number of RM detections spanning a long timescale are accumulated, the orbital period could be derived from these RM data. So we plan to observe FRB 20201124A and FRB 20190520B for a long time.”
While the new research offers a compelling explanation for the source of FRB 20201124A, FRBs are not a one-size-fits-all phenomenon. Because these signals can be so different, scientists think they are likely caused by a variety of strange astrophysical objects, all of which must be extremely powerful in order to be seen across millions, and even billions, of light years.
With that in mind, it will take many more years to identify the sources of all FRBs, and some may remain unexplained forever. Still, scientists plan to learn as much as possible about these strange signals from space, both to sate their curiosity and also because FRBs can help shed light on longstanding cosmic mysteries, such as the rate at which the universe is expanding and the reason it appears to be missing some forms of matter.
“FRBs are important cosmological probes,” Wang concluded.