Polarized light provides fresh insight into mysterious radio bursts

Astronomers have been puzzling over the origin of mysterious radio bursts (FRBs) since they were spotted in 2007. Researchers now have their first look at FRBs that do not repeat, that is, those that have only produced a single burst light until today. . The authors of a new paper published in The Astrophysical Journal looked specifically at the properties of the polarized light emitted by these FRBs, providing further insight into the origin of the phenomenon. The analysis supports the hypothesis that there are different origins for recurrent and non-recurrent FRBs.

“This is a new way to analyze the data we have on FRBs. Instead of just looking at how bright something is, we’re also looking at the angle of the light’s vibrating electromagnetic waves,” said co-author Ayush Pandhi, a graduate student at the University of Toronto’s Dunlap Institute for Astronomy and Astrophysics. “It gives you additional information about how and where that light is produced and what it went through on its journey to us over many millions of light years.”

As we’ve previously reported, FRBs involve a sudden burst of radio frequency radiation lasting just a few microseconds. Astronomers have over a thousand of them to date; some come from sources that continuously emit FRBs, while others appear to burst once and fall silent. You can produce this kind of sudden energy surge by destroying something. But the existence of repeated sources suggests that at least some of them were produced by an object that survives the event. This has led to a focus on compact objects such as neutron stars and black holes – particularly a class of neutron stars called magnetars – as possible sources.

There have also been many FRBs discovered that do not appear to repeat at all, suggesting that the conditions that produce them may destroy their source. This is consistent with a storm – a strange astronomical event caused by the sudden collapse of an extremely massive neutron star. The event is triggered by a previous merger of two neutron stars; this creates an unstable intermediate neutron star, which does not immediately collapse from its rapid rotation.

In a storm, the neutron star’s strong magnetic fields slow its rotation, causing it to collapse into a black hole hours after merging. This collapse suddenly wipes out the dynamo that powers the magnetic fields, releasing their energy in the form of a fast radio burst.

So the events we have lumped together as FRBs may actually be the product of two different events. Repeated events occur in the environment around a magnetar. A shock event is caused by the death of a highly magnetized neutron star within a few hours of its formation. Astronomers announced the discovery of a possible storm that could be associated with an FRB last year.

Only about 3 percent of FRBs are of the repeating variety. According to Pandhi, this is the first analysis of the other 97 percent of FRBs that do not repeat, using data from Canada’s CHIME (Canadian Hydrogen Intensity Mapping Experiment) instrument. CHIME was built for other observations, but is sensitive to many of the wavelengths that make up an FRB. Unlike most radio telescopes, which focus on small spots in the sky, CHIME scans a large area, allowing it to pick out FRBs even though they almost never occur in the same place twice.

Pandhi etc. decided to investigate how the direction of polarization of light from 128 non-repeating FRBs changes to learn more about the environments from which they originate. The team found that the polarized light from non-repeating FRBs varies both over time and with different colors of light. They concluded that this particular sample of nonrecurring FRBs is either a distinct population or more evolved versions of these types of FRBs that are part of a population that originated in less extreme environments with lower explosion rate. This is consistent with the notion that non-repeating FRBs are quite different from their rarer repeating FRBs.

The Astrophysical Journal, 2024. DOI: 10.3847/1538-4357/ad40aa (About DOIs).