A baby shower in a retirement home – that would surely raise some eyebrows. Likewise, astronomers were surprised to find a fast radio burst in a globular cluster. Astronomers think the enigmatic, millisecond-duration flashes of radio waves arise on newborn neutron stars. However, the stars in globular clusters are almost as old as the universe itself.
“This is a really astonishing result,” says Nanda Rea (Institute of Space Sciences, Spain), who was not involved in the study. “What is a young neutron star doing in a globular cluster?”
Fast radio bursts (FRBs) were first discovered in 2007. In about one-thousandth of a second, they release as much energy as the Sun does in days. Throughout the observable universe, many hundreds of FRBs go off every single day, but they are hard to catch. We don’t know where or when the next one will occur, and most radio telescopes have a very small field of view, as if we were watching the universe through a drinking straw. (The exception is the Canadian CHIME telescope, whose huge field of view has enabled the detection of hundreds of FRBs to date.)
Fortunately, some FRBs are repeaters – a sure sign that the “engine” that produces the bursts isn’t completely destroyed in the event. With a bit of patience and luck, astronomers can study multiple bursts from the same source, just by aiming their instruments on the location of one of the two-dozen known repeaters.
That’s just what a large collaboration led by Franz Kirsten (Onsala Space Observatory, Sweden) did last year. Using the 12 radio telescopes of the European Very Long Baseline Interferometry (VLBI) Network, they observed the location of FRB 20200120E, a fast radio burst in the outskirts of the spiral galaxy M81 some 12 million light-years away in Ursa Major. “This is the nearest extragalactic FRB known to date,” says Kirsten. (In 2020, astronomers detected a low-luminosity FRB in our own Milky Way.) Between February and April 2021, the team detected a total of five bursts.
Using interferometry, the team pinpointed the position of the repeater on the sky to a precision of a mere 1.25 milli-arcsecond. They were surprised to see that the energetic flashes originated in one of M81’s many globular clusters. In today’s Nature (preprint available here), the researchers claim that the probability of a chance alignment is less than one in 6,000, concluding that the association is “robust.”
And that’s where the puzzle starts. There’s ample evidence that fast radio bursts originate on or around magnetars – newborn and highly magnetized neutron stars.
But that would imply a relatively recent supernova explosion in the globular cluster: “The most common and likely scenario for the formation of magnetars is the core-collapse supernova one,” says Kirsten.
However, core-collapse supernovae are the terminal detonations of massive stars. And since massive stars have short lifetimes, you don’t expect supernova explosions in globular clusters: Globulars formed in the early youth of the universe, so any massive stars they may have contained would have gone supernova many billions of years ago.
The most likely explanation, according to the researchers, is that the magnetar formed in another way. Since globular clusters have a high stellar density, they are expected to contain many tight binary stars. If a white dwarf siphoned mass off a stellar companion, it might have gained enough to collapse into a neutron star.
Another alternative is the merger of, say, two white dwarfs, a white dwarf and a neutron star, or maybe even two low-mass neutron stars. Any such collision could also lead to further collapse and the birth of a highly magnetized neutron star.
But Rea, who is an expert on magnetars, says there’s no unambiguous evidence that neutron stars can form through accretion-induced collapse or merger-induced collapse. “So far, it’s only theory,” she says, adding that this particular FRB might have come from something else altogether.
Fast radio bursts in globular clusters may turn out to be common. FRB 20200120E is “quite a low-luminosity FRB,” Kirsten notes. “It would not have been detected if it were at the distance of the next closest FRB.” According to Rea, “There might be many of them. It’s difficult to say on the basis of just one example.”
Rapid Flickering in the Burst
In another paper published today in Nature Astronomy (preprint available here), Kenzie Nimmo (ASTRON Netherlands Institute for Radio Astronomy) and colleagues present evidence for extremely brief “sub-bursts” in the same FRB: isolated shots of emission that last a mere 60 nanoseconds. Such short flashes might indicate an incredibly small emitting region – maybe just a few dozen meters across.
The powerful sub-bursts resemble nano-shots, a subset of the brief giant radio pulses observed from the Crab pulsar, a neutron star less than 1,000 years old. The similarity suggests a link between young pulsars and fast radio bursts. Magnetic twists and snaps in neutron star magnetospheres might play a role in both phenomena. However, the precise emission mechanism of the Crab’s nano-shots and the rapid flickering of FRB 20200120E is still unknown.
One thing is clear, though: Fast radio bursts are a diverse bunch. They span a wide range of radio luminosities and have been discovered in massive star-forming galaxies, in small dwarf galaxies, and in mid-size spirals — in stellar nurseries but also in stellar retirement homes. Apparently, the engines that power them can be born in a variety of ways.
As team member Jason Hessels (ASTRON) says, “I would be surprised if FRBs stop surprising us. We’re still very much in the discovery phase of understanding what these sources are.”