Researchers aim for the source of the fast radio burst

Enlarge / An event on the surface of a magnetar can produce a rapid radio explosion.

A fast radio burst is exactly what the name suggests: a sudden wave of photons at radio frequencies that often lasts less than a second. After the scientists finished convincing themselves that they saw no equipment interference, a search was carried out for what generated the large amounts of energy involved in fast radio bursts (FRBs).

The discovery of the first iterative FRB tells us that the process that generates the FRB does not destroy the object that generated it. Eventually, FRBs were discovered that were associated with events at additional wavelengths, allowing their source to be identified: magnetars, the subset of neutron stars that have the most extreme magnetic fields in the Universe. While that shows excellent progress, it still doesn’t tell us anything about the physics of how the explosion was generated—knowledge that will probably tell us why most magnetars don’t and why they tend to start and stop so suddenly.

Now, researchers have identified FRBs that help limit our ideas of what can produce them. The FRB itself appears to be a single event, but consists of nine individual explosions separated by about 215 milliseconds. The fast speed means that the source of the explosion is almost certainly near the surface of the magnetar.

Bursts and sub-bursts

New work comes out of the Canadian CHIME instrument, which was built for other observations but found to be sensitive to many of the wavelengths that make up the FRB. CHIME scans a large area of ​​the sky, allowing it to select FRBs despite the fact that they almost never occur in the same place twice.

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The automated analysis pipeline that selects potential FRB events should have missed an event called FRB 20191221A, simply because it was much longer than the FRB as defined, taking nearly three seconds to increase radio emissions and then drop back down. to the background level again. But the data is kept for future analysis because those three seconds appear to contain multiple independent bursts, and it is these sub-bursts that trigger the system to flag the data.

The individual bursts in this event are seen at various wavelengths.

Although we have identified recurring sources previously, which produce single explosions with long separations between them. In contrast, FRB 20191221A only has a range of about 215 milliseconds.

In fact, the gap between these sub-bursts is very regular. Researchers estimate the odds of detecting something this seemingly ordinary without actually being regular as one in 10^-11gives them “high confidence” that the signal is periodic.

Since that incident, there has been no sign that there will be another event from the same region as FRB 20191221A. It also appears to have come from sources outside our galaxy.

Close to the core

But it’s really the periodicity that tells us something about the nature of the FRB. Neutron stars themselves are extremely extreme environments, so their surfaces can generate the kind of extreme energy required for FRBs. But magnetars have extreme magnetic fields that extend their high-energy environment well beyond the surface of the neutron star. (Their field strength is so strong that the normal orbitals of the atoms are distorted, preventing chemistry from happening near them.) So it’s not clear how close the resulting neutron star FRBs are.

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The timing of these sub-bursts strongly argues that they occur on the surface of stars. The millisecond-level separation between events is consistent with the rotational speed of the neutron stars we see in many pulsars. So what we see with FRB 20191221A may be a widespread event on the surface of a neutron star that creates a beam that flashes across Earth with the star’s rotation before fading back. Given the length of the pulses, however, the source must be much wider than the pulsars we observed.

An alternative explanation is that the star is rotating slowly, and we are witnessing an event that makes its crust vibrate, with bursts of emission adjusted to the frequency of the crust’s vibrations. Again, the extreme nature of the neutron star means that a “star quake” will have far more energy than we’ve ever seen on Earth.

On the other hand, it’s hard to understand how you can produce this kind of periodicity at a distance from the magnetar without having a periodic source in the star itself.

However, all of this is based on the assumption that the 20191221A FRB is more representative of the FRB in general. By searching the CHIME data, the research team has found two examples of what appears to be similar periodicity but with a lower number of sub-bursts. However, in part due to the smaller number of repetitions, statistical certainty about whether they have regular splits is much lower.

So, while there is still some uncertainty about how representative the 20191221A FRB is, it is progress that is slowly getting us closer to understanding the FRB over the last decade. By gradually narrowing the number of possible explanations, we are slowly getting closer to understanding what produced this extreme event.

Nature, 2022. DOI: 10.1038/s41586-022-04841-8 (About DOI).

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