Over the last 15 years, mysterious radio bursts — first from other galaxies, and then our own — have puzzled astronomers from around the world by repeating irregularly with no clear mechanism to cause it. Were they the collision of black holes? Blasts from supernovae? Ultra-magnetized neutron stars? Aliens?
Two papers released this month dealing with one of these fast radio bursts (or FRBs) might have an answer — for at least some of them. In Nature, a team argued that one recently detected FRB is from an unusual spot with an unusual nature. In Nature Communications, another team analyzed its oddities to determine that that signal is coming not from one star, but from two.
What’s new — Fast Radio Burst 20201124A — as its name might suggest — first showed up on November 24, 2020, at the massive Five-hundred Meter Aperture Spherical Telescope. FAST, which is nestled in the mountains of southwest China, is the world’s biggest filled-in radio telescope by a large margin and has been instrumental in better understanding FRBs.
All fast radio bursts are unusual, but FRB 20201124A was an especially odd one. First of all, its polarization and signal strength swung wildly, even over the course of just one single day. Over the course of 82 hours of observation spread over two months, according to the work published in Nature, FAST detected 1,863 different bursts.
Speaking with Inverse, lead author Fayin Wang of Nanjing University noted that this is the first FRB to show these kinds of variations in its radio waves. It’s the widest burst yet seen, its signal is circularly polarized, and the rotation of its signal “had a short-time variation during the first 36 days of FAST observations, followed by a constant RM during the later 18 days” — meaning the signal was interfered with, then free from interference.
Digging into the details — Polarization can tell astronomers a lot about FRBs. The degree of polarization in the waves helps to show what it might have passed through on the way to being picked up on a radio telescope, like what makes up the cosmic web between galaxies.
Beyond this, FRB 20201124A wasn’t coming from the usual place for a fast radio burst. Instead of coming from a new magnetar formed by the explosion of a supernova or at the center of a galaxy, this signal came from a relatively empty area just outside the arms of a barred spiral galaxy — the same sort of galaxy as the Milky Way.
So for the last two years, this has been an especially mysterious fast radio burst. But Wang and his team believe they understand what’s causing 20201124A’s strange behavior. This fast radio burst was caused not by just a lone super-magnetized neutron star, but by a super-magnetized neutron star and its larger buddy, a Be star — a blue giant star with strange spectral lines corresponding to rapid rotation.
Magnetars are physically tiny — only about 20 kilometers across — but have more mass than the Sun, and extremely strong magnetic fields, about a trillion times stronger than the Earth’s. Be stars, on the other hand, are extremely bright, extremely blue, much larger than the Sun, and are spinning fast enough that they eject a disc of hydrogen around themselves.
Based on their modeling, what the astronomers think happened is this: the magnetar and the Be star were once a binary system. When the soon-to-be magnetar went supernova, the tremendous amount of energy released knocked the two out of alignment, so the magnetar passes through the disk twice every approximately 80 days.
As the magnetar approaches periastron — its nearest approach to the Be star — it passes behind the disc, which in turn interferes with the radio signals it gives off. This would explain the strange signals coming from 20201124A.
Why it matters — Since the first FRB was discovered by an Australian telescope in 2007, the cause of the phenomenon has been unclear. Some of them repeat regularly, some of them spit out signals at random. Many of them seem to be magnetars, but others may be the result of white dwarves. This result is also not the only possible source of FRBs from a binary system — in the past, other researchers have proposed some of them may come from the collapse and merger of binary stars.
Binary pairs might not explain all these mysterious signals, but FSB20201124A’s origin isn’t alone. Of the about 30 magnetars we know about, another one is already suspected to be in a binary system as well.
The existence and the patterns of FRBs are still mysterious, but Wang notes that this model “can also naturally explain the RM variations of other FRBs” that have already been discovered. These include 2019’s FRB 20190520B and 2018’s FRB 20180916B. This may not be the explanation for all these bursts — maybe alien signals still have a chance? — but these results show how extremely complex local environments produce these strange signals.