Anomalous Fast Radio Bursts from other galaxies could help astronomers study dark matter

The science of Fast Radio Bursts (FRBs), anomalous flashes of radio signals appearing in the sky roughly once per minute, just got a little clearer — if also a little weirder.

A new study appearing Wednesday in the journal Nature has tracked a repeating FRB with pinpoint accuracy to its host galaxy and adds evidence to the hypothesis that FRBs are powered by magnetars — neutron stars with immensely energetic magnetic fields.

But the source of the FRB, a globular cluster of old stars, is not where astronomers would expect to find a magnetar, suggesting a new route to the formation of such a powerful stellar dynamo, or a non-magnetar source of the radio bursts coming from what is known as FRB 20200120E.

“Either this is a magnetar and we sort of have the proof that this other [magnetar] formation channel does exist,” said Franz Kirsten, an astronomer at the Chalmers University of Technology and first author of the paper. Or “if it’s not a magnetar, then it just adds to the diversity of sources that can generate fast radio bursts”.

FRBs are a newer phenomenon in astronomy, with the first detected in 2007, and the first localised to the Milky Way galaxy discovered in 2020. FRB signals are extremely short — on the order of milliseconds — and release a tremendous amount of energy in those short bursts has led astronomers to hypothesize they originate in magnetars, which possess the most energy-dense magnetic fields in the cosmos.

But it’s still just a hypothesis. Since most FRBs discovered so far are extragalactic and far away, and given the relatively short time they have been studied, astronomers are still trying to understand their basic nature, according to Vikram Ravi, a professor of astronomy at Caltech and author of a commentary appearing alongside Kirsten’s paper in Nature.

“We’re trying to piece together these really sort of disparate, and in some ways almost circumstantial pieces of evidence to try to find a picture of what these are,” Ravi said. “A process of trying to form the picture through this immense sort of fog of all the things between us and the FRBs.”

That’s what got Dr Kirsten and his colleagues excited when the Canadian Canadian Hydrogen Intensity Mapping Experiment (Chime) telescope placed FRB 20200120E somewhere in the Messier 81 galaxy, which is only 11 and a half light years away.

“It is the closest extragalactic fast radio burst by far,” he said, “so 40 times closer than the next closest.”

By training an array of 11 radio telescopes at the radio bursts of FRB 20200120E, Dr. Kirsten and his colleagues were able to identify its source as a globular cluster away from the center of its host galaxy. Which, he added, is somewhat weird.

“A globular cluster is a system of very old stars, and all these massive stars, they’re long gone,” Dr Kirsten said. “They live a very short life and they explode within a couple of million years.”

But the leading theory for how magnetars form is that they result from a core collapse supernova — a star roughly eight to 10 times more massive than our Sun goes nova and leaves behind a young neutron star or magnetar. But candidate stars for such a collapse in the globular cluster hosting FRB 20200120E are already dead.

“You quickly come to the conclusion that if this is a magnetar in that globular cluster, it can not have formed through core collapse supernovae,” Dr Kirsten said. But there is a longstanding alternative theory of magnetar formation where a white dwarf star steals enough mass from a nearby companion star that the white dwarf bloats up to the point it can no longer support itself, “and then it collapses and informs a neutron star, or a magnetar in this case, which is what we think happened.”

It’s not yet certain that FRB 20200120E originates in a magnetar. Dr Kirsten notes that certain types of pulsars could also generate a similar radio burst, as might a low mass X-ray binary system, a black hole, or neutron star with a binary companion with intensely interactive magnetic fields. So far, FRB 20200120E has displayed no X-ray activity, but he notes current X-ray observatories are not sensitive enough to rule a low mass X-ray binary system out. That may require new, more sensitive satellites.

In the meantime, FRB 20200120E suggests astronomers can turn from trying to figure out what FRBs actually are, to using them to better understand other phenomena. Dark matter, for example.

“About 80 to 90% of the matter that we know is out that is not seen, it doesn’t shine,” Dr Ravi said. “And yet, we know it’s there from the influence of gravity, as well as our understanding of cosmology.”

But although we cannot easily observe dark matter, the radio signals emitted by FRBs should be delayed by different amounts and at different frequencies as they pass through dark matter, or follow slightly deviated paths due to the gravitational fields of dark matter objects between Earth and an FRB.

“What we hope to do is use FRBs to probe the content and distribution and physical conditions in this otherwise unseen matter,” Dr Ravi said. “There are several models of dark matter that say that a good fraction of dark matter is just these sort of little free floating black holes all over the universe, and we’d love to be able to use FRBs to test that assertion.”