Fast radio bursts originate near the surface of stars

When fast radio bursts (FRBs) were first detected in 2007, they were a complete enigma. As their name implies, these events involve a very brief eruption of radio emissions and then typically silence, though a few objects appear to be capable of sending out multiple bursts. By obtaining enough data from lots of individual bursts, researchers gradually put the focus on magnetars, versions of neutron stars that have intense magnetic fields.

But we still don't know whether a magnetar is a requirement for an FRB or if the events can be triggered by less magnetized neutron stars as well. And we have little hint of the mechanism that produces the burst itself. Bursts could potentially be produced by an event in the star's magnetic field itself, or the star could be launching some energetic material that subsequently produces an FRB at some distance from the star.

But now, a rare burst has provided indications that FRBs likely originate near the star and that they share a feature with the emissions of pulsars, another subtype of neutron star.

A lucky sighting

Both of these conclusions are based on the observation of a single FRB, termed FRB 20221022A, that was detected in October 2022. It was picked up by CHIME (Canadian Hydrogen Intensity Mapping Experiment). That observatory was built to look at radio emissions from different sources, but it turned out to be capable of staring at a very broad chunk of the sky and extremely good at identifying FRBs.

The event was relatively easy to localize, allowing it to be associated with a specific galaxy that is relatively nearby, at least in cosmological terms (about 200 million light-years away). Its proximity was critical for a couple of reasons. First, it meant that the radio waves that arrived at Earth were intense enough to allow various analyses of the properties of the burst. The second is that the photons of the burst spent relatively little time traveling in the space between galaxies. Instead, most of the influence matter has had on these photons comes from materials inside a galaxy—either the galaxy that hosts the source or within our own Milky Way.

One of the two papers published on Wednesday looks at the polarization of the photons in the burst itself, finding that the angle of polarization changes rapidly over the 2.5 milliseconds that FRB 20221022A lasted. The 130-degree rotation that occurred follows an S-shaped pattern, which has already been observed in about half of the pulsars we've observed—neutron stars that rotate rapidly and sweep a bright jet across the line of sight with Earth, typically multiple times each second.

The implication of this finding is that the source of the FRB is likely to also be on a compact, rapidly rotating object. Or at least this FRB. As of right now, this is the only FRB that we know displays this sort of behavior. While not all pulsars show this pattern of rotation, half of them do, and we've certainly observed enough FRBs we should have picked up others like this if they occurred at an appreciable rate.

Scattered

The second paper performs a far more complicated analysis, searching for indications of interactions between the FRB and the interstellar medium that exists within galaxies. This will have two effects. One, caused by scattering off interstellar material, will spread the burst out over time in a frequency-dependent manner. Scattering can also cause a random brightening/dimming of different areas of the spectrum, called scintillation, and somewhat analogous to the twinkling of stars caused by our atmosphere.

In this case, the photons of the FRB have had three encounters with matter that can induce these effects: the sparse intersteller material of the source galaxy, the equally sparse interstellar material in our own Milky Way, and the even more sparse intergalactic material in between the two. Since the source galaxy for FRB 20221022A is relatively close to our own, the intergalactic medium can be ignored, leaving the detection with two major sources of scattering.