From Swinging to Twinkling: How Polarization and Scintillation Revealed FRB 20221022A’s Origins

FRB 20221022A, spotted by the CHIME/FRB project, is no ordinary fast radio burst. Through a bizarre twist of polarization (and fate), this nearby FRB also displays a curious scintillation pattern that hints at its origin inside the intense magnetic field environment of a neutron star.
Published in Astronomy
From Swinging to Twinkling: How Polarization and Scintillation Revealed FRB 20221022A’s Origins
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Fast Radio Bursts (FRBs) are among the most mysterious and exciting phenomena in the universe. These incredibly brief flashes of radio waves, lasting just milliseconds—shorter than the blink of an eye—release as much energy as the Sun produces in three days. The immense energy of these events is the reason why FRBs can be observed at incredible distances, billions of light-years away! While their origins remain elusive, some FRBs are thought to come from magnetars—highly magnetized, explosive remnants of massive stars. A notable breakthrough came when CHIME/FRB and STARE2 detected a burst from SGR 1935+2154, a known magnetar in our galaxy, conclusively linking FRBs to extragalactic magnetars. Yet, we still don't fully understand the physical processes that create these cosmic signals.

We are both members of the Canadian Hydrogen Intensity Mapping Experiment FRB (CHIME/FRB) collaboration, currently the world-leading FRB discovery machine. CHIME isn’t just great at picking up loads of FRBs to study the big picture—it’s also amazing at spotting the oddballs, the ones that really stand out. And it turns out, these misfits are super important! They help us figure out the limits of what FRBs can be, giving scientists lots of clues for building theories about where these signals come from, whether they may arise from multiple sources, and what extreme physics is responsible. When we first discovered FRB 20221022A, it seemed pretty ordinary, as far as FRBs go. But once we dug into its polarization properties, that’s when its uniqueness really came to light.

Most FRBs we’ve found so far don’t change much in their polarized signal, but FRB 20221022A was totally different. Over its brief 2-millisecond burst, the polarization didn’t just shift—it evolved in a really structured, deliberate way. In fact, it looked strikingly like the S-shaped swing you see in the signals from radio pulsars, those neutron stars hanging out in our own Milky Way. So similar, in fact, that it raised the question... could this FRB actually be a pulse from a nearby pulsar in disguise?

To confirm that the source was indeed an FRB, we first needed to establish that the signal was coming from outside of our galaxy. One way to do this is to localize the position of the signal on the sky with sufficient accuracy to identify a host galaxy from which the signal originated. Most of the time, CHIME/FRB localizations are just too big to nail down a single host galaxy for an FRB—there are usually many candidates. But that’s about to change. The CHIME/FRB team is building Outrigger stations, which will let us pinpoint the location of each FRB CHIME detects with incredible precision, right down to within its host galaxy, using a technique called very long baseline interferometry. So, stay tuned—there’s some really exciting science coming from that project in the near future.

For this FRB, though, we got lucky. It came from a massive, relatively nearby galaxy, so we could confidently match it to its host. That also let us rule out the idea of this being a sneaky Galactic pulsar in disguise.

So what does the polarization behavior mean? We think the swinging polarization angle comes from a beam sweeping across our line of sight, like a lighthouse. In the case of pulsars, such behavior is common and is understood to be an artifact of the changing angle of magnetic field lines projecting from poles of the neutron star as it rotates. While the source of FRB 20221022A cannot conclusively be shown to be a neutron star, the polarization swing tells us that the FRB emission is locked to the rotation of some object, meaning the source has to be incredibly close to that rotating object.

There’s been a big debate in the field about whether FRB signals can even make it out of the intense plasma environments close to magnetars. This has split emission models into two camps: the “nearby” ones, where the signal forms within the star’s magnetic environment, and the “far-away” ones, where it happens much farther out, driven by a shock launched from the star. The polarization behavior of this FRB is a win for the nearby models—it’s strong evidence that the signal originated close to the central star.

While we were working on this polarization result, our collaborator Pawan Kumar reached out with an exciting theoretical idea he’d been developing: how scintillation could help distinguish between competing FRB emission models. The concept is surprisingly intuitive. Picture the night sky—you see stars “twinkling”, thanks to scintillation caused by Earth’s atmosphere. But planets, those bright spots that don’t twinkle, stand out. The reason for this difference is that stars are much farther away, so they appear as point sources compared to the diffractive scale of the atmosphere, while planets are extended sources. This simple observation ties scintillation to the apparent size of astronomical objects.

Pawan realized we could use the same principle to probe FRB emission models. Different models predict different sizes for the emission region, and scintillation—this time caused by plasma in the FRB's host galaxy—could act as a natural astrophysical lens. However, so far, all scintillation observed in FRBs comes from our own Milky Way’s interstellar medium. That sent us on a mission: could we find evidence of extragalactic scintillation in CHIME/FRB data?

This idea aligned perfectly with discussions we’d been having for our polarization paper. FRB 20221022A had a clean, unscattered profile—scatter broadening was negligible, which theoretically should make scintillation more pronounced. It stood out as a prime candidate for hunting down this elusive extragalactic effect.

And what a candidate it was. Not only did we discover extragalactic scintillation in an FRB for the first time, but we also used it to measure the size of the emission region, which we presented in a companion paper in the same Nature volume. The result? The emission region must be smaller than 30,000 km—far too compact for shock models, confirming that FRB 20221022A originates from within the magnetosphere of its compact object.

An artist's illustration of a neutron star in a distant galaxy emitting a radio beam from within its magnetic environment, that rotates with the star. As the radio waves travel through dense plasma within the galaxy, they split into multiple paths, causing the observed signal to flicker in brightness. Credit: Daniel Liévano.

For years, there’s been a heated debate in the FRB field: can the incredibly luminous bursts we detect actually escape from the extreme plasma within the magnetic environment of a compact object? With FRB 20221020A, we’ve tackled this head-on. By combining polarization and scintillation analyses, we’ve shown that this FRB does indeed originate from such an extreme plasma environment—yet it’s still detectable by CHIME on Earth. This points to a gap in our understanding of how these extreme plasmas behave. We’re talking about the most intense magnetic fields in the Universe, a realm so extreme it’s hard to wrap your head around. 

What’s fascinating about FRB 20221020A is that it’s a single, non-repeating FRB with a polarization signature that stands out from the broader FRB population. This raises the tantalizing possibility that different emission mechanisms could be at play across the diverse population of FRBs to explain the vast diversity we see in their properties

Looking ahead, we’re excited to expand this approach—using polarization and scintillation as powerful tools to probe the origins of FRBs—on a much larger sample. Who knows what surprises await us in this wildly diverse and still-mysterious field?

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