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Polarized light yields fresh insight into mysterious fast radio bursts

Artist’s rendition of how the angle of polarized light from an FRB changes as it journeys through space.
Enlarge / Artist’s rendition of how the angle of polarized light from a fast radio burst changes as it journeys through space.

CHIME/Dunlap Institute

Astronomers have been puzzling over the origins of mysterious fast radio bursts (FRBs) since the first one was spotted in 2007. Researchers now have their first look at non-repeating FRBs, i.e., those that have only produced a single burst of light to date. The authors of a new paper published in The Astrophysical Journal looked specifically at the properties of polarized light emitting from these FRBs, yielding further insight into the origins of the phenomenon. The analysis supports the hypothesis that there are different origins for repeating and non-repeating 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 has passed through on its journey to us over many millions of light years.”

As we’ve reported previously, FRBs involve a sudden blast of radio-frequency radiation that lasts just a few microseconds. Astronomers have over a thousand of them to date; some come from sources that repeatedly emit FRBs, while others seem to burst once and go silent. You can produce this sort of sudden surge of energy by destroying something. But the existence of repeating sources suggests that at least some of them are produced by an object that survives the event. That has led to a focus on compact objects, like neutron stars and black holes—especially a class of neutron stars called magnetars—as likely sources.

There have also been many detected FRBs that don’t seem to repeat at all, suggesting that the conditions that produced them may destroy their source. That’s consistent with a blitzar—a bizarre astronomical event caused by the sudden collapse of an overly massive neutron star. The event is driven by an earlier merger of two neutron stars; this creates an unstable intermediate neutron star, which is kept from collapsing immediately by its rapid spin.

In a blitzar, the strong magnetic fields of the neutron star slow down its spin, causing it to collapse into a black hole several hours after the merger. That collapse suddenly deletes the dynamo powering the magnetic fields, releasing their energy in the form of a fast radio burst.

So the events we’ve been lumping together as FRBs could actually be the product of two different events. The repeating events occur in the environment around a magnetar. The one-shot events are triggered by the death of a highly magnetized neutron star within a few hours of its formation. Astronomers announced the detection of a possible blitzar potentially associated with an FRB last year.

Only about 3 percent of FRBs are of the repeating variety. Per Pandhi, this is the first analysis of the other 97 percent of non-repeating FRBs, using data from Canada’s CHIME instrument (Canadian Hydrogen Intensity Mapping Experiment). 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 points in the sky, CHIME scans a huge area, allowing it to pick out FRBs even though they almost never happen in the same place twice.

Pandhi et al. decided to investigate how the direction of the light polarization from 128 non-repeating FRBs changes to learn more about the environments in which they originated. The team found that the polarized light from non-repeating FRBs changes both with time and with different colors of light. They concluded that this particular sample of non-repeating FRBs is either a separate population or more evolved versions of these kinds of FRBs that are part of a population that originated in less extreme environments with lower burst rates. That’s in keeping 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).

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