A fast radio burst’s unlikely source may be a cluster of old stars

In a galaxy not so far away, astronomers have located a surprising source of a mysterious, rapid radio signal.

The signal, a repeating fast radio burst, or FRB, was observed over several months in 2021, allowing astronomers to pinpoint its location to a globular cluster — a tight, spherical cluster of stars — in M81, a massive spiral galaxy 12 million light-years away. The findings, published February 23 in Nature, are challenging astronomers’ assumptions of what objects create FRBs.

“This is a very revolutionary discovery,” says Bing Zhang, an astronomer at the University of Nevada, Las Vegas who was not involved in the study. “It is exciting to see an FRB from a globular cluster. That is not the favorited place people imagined.”

Astronomers have been puzzling over these mysterious cosmic radio signals, which typically last less than a millisecond, since their discovery in 2007 (SN: 7/25/14). But in 2020, an FRB was seen in our own galaxy, helping scientists determine one source must be magnetars — young, highly magnetized neutron stars with magnetic fields a trillion times as strong as Earth’s (SN: 6/4/20).

The new findings come as a surprise because globular clusters harbor only old stars — some of the oldest in the universe. Magnetars, on the other hand, are young leftover dense cores typically created from the death of short-lived massive stars. The magnetized cores are thought to lose the energy needed to produce FRBs after about 10,000 years. Globular clusters, whose stars average many billions of years old, are much too elderly to have had a sufficiently recent young stellar death to create this type of magnetar. 

To pinpoint the FRB, astronomer Franz Kirsten and colleagues used a web of 11 radio telescopes spread across Europe and Asia to catch five bursts from the same source. Combining the radio observations, the astronomers were able to zero in on the signal’s origins, finding it was almost certainly from within a globular cluster.

“This is a very exciting discovery because it was completely unexpected,” says Kirsten, of ASTRON, the Netherlands Institute for Radio Astronomy, who is based at the Onsala Space Observatory in Sweden.

The new FRB might still be caused by a magnetar, the team proposes, but one that formed in a different way, such as from old stars common in globular clusters. For example, this magnetar could have been created from a remnant stellar core known as a white dwarf that had gathered too much material from a companion star, causing it to collapse.

“This is a [magnetar] formation channel that has been predicted, but it’s hard to see,” Kirsten says. “Nobody has actually seen such an event.”

Alternatively, the magnetar could have been formed from the merger of two stars — such as a pair of white dwarfs, a pair of neutron stars or one of each — in close orbit around one another, but this scenario is less likely, Kirsten says. It’s also possible the FRB source isn’t a magnetar at all but a very energetic millisecond pulsar, which is also a type of neutron star that could be found in a globular cluster, but one that has a weaker magnetic field.

To date, only a few FRB sources have been precisely pinpointed, and their locations are all in or close to star-forming regions in galaxies. Besides adding a new source for FRBs, the findings suggest that magnetars created from something other than the death of young stars might be more common than expected.