Rapid radio bursts are exactly what their name implies: a sudden surge of photons at radio frequencies that often lasts less than a second. Once scientists convinced themselves that they weren’t looking at equipment failures, they looked for what produced the vast amounts of energy involved in a fast radio burst (FRB).
The discovery of the first repeating FRB told us that the process that generates an FRB does not destroy the object that does the production. Finally, an FRB associated with events at extra wavelengths was found, identifying the source: a magnetar, a subset of neutron stars with the most extreme magnetic fields in the universe. While that represents excellent progress, it still doesn’t tell us anything about the physics of how the burst is produced — knowledge that would presumably tell us why most magnetars don’t produce them and why the burst tends to start and stop so suddenly. .
Now, researchers have identified an FRB that helps narrow down our ideas of what can produce them. The FRB itself appears to be a single event, but it is composed of nine separate bursts separated by approximately 215 milliseconds. The high tempo means that the burst’s source should almost certainly be close to the magnetar’s surface.
Bursts and Subbursts
The new work comes from Canada’s CHIME instrument, which was built for other observations but has been shown to be sensitive to many of the wavelengths that make up an FRB. CHIME scans a huge area of the sky, allowing it to distinguish FRBs, despite the fact that they almost never appear twice in the same place.
The automated analysis pipeline that sorts out potential FRB events should have missed an event called FRB 20191221A simply because it was much longer than FRBs as they are defined, and it takes nearly three seconds for radio emissions to increase and then fall again. back to background levels. But the data has been saved for future analysis because those three seconds seem to contain several independent bursts, and these subbursts prompted the system to flag the data.
The individual bursts within this event are visible over a wide range of wavelengths.
Although we previously identified repeating sources, they produced single bursts with a long separation between them. FRB 20191221A, on the other hand, had a separation of only about 215 milliseconds between them.
In fact, the gaps between these sub-bursts were remarkably regular. The researchers estimate the chance of detecting something that looks so regular, without actually being regular, as one in 10-11giving them “high confidence” that the signal is periodic.
Since that event, there is no sign of any other event from the same region as FRB 20191221A. It also appears to come from a source outside our galaxy.
Close to the core
But it’s really the periodicity that tells us something about the nature of FRBs. Neutron stars themselves are very extreme environments, so their surfaces can produce the kind of extreme energy needed for an FRB. But magnetars have extreme magnetic fields that extend the high-energy environment far beyond the neutron star’s surface. (The strength of their fields is so strong that the normal orbitals of atoms are distorted, preventing chemistry from happening nearby.) So it’s not clear how close to the neutron star FRBs are generated.
The timing of these sub-bursts strongly proves that it is on the surface of the star. The millisecond-level separation between events corresponds to the rotational speed of neutron stars that we see on many pulsars. So what we’re seeing with FRB 20191221A could be a broad event on the neutron star’s surface that creates a beam that flickers across the Earth with the star’s rotation before fading again. However, given the length of the pulses, the source should have been much wider than any pulsar we have observed.
An alternative explanation could be that the star is rotating slowly and that we are looking at an event that has caused the crust to vibrate, where the burst of emissions is tuned to the vibrational frequency of the crust. Again, the extreme nature of neutron stars means that a “star quake” would have much more energy than we would ever see on Earth.
In contrast, it is difficult to understand how to generate this kind of periodicity distant from the magnetar without a periodic source on the star itself.
However, all of this is based on the assumption that FRB 20191221A is representative of FRBs in general. By sifting through CHIME data, the research team came up with two examples of what appears to be a similar periodicity, but with a lower number of sub-bursts. However, partly due to the smaller number of repetitions, the statistical certainty whether they have a regular separation is much lower.
So while there is still some uncertainty about how representative FRB 20191221A is, this is the kind of progress that has slowly brought us closer to understanding FRBs over the past decade. By gradually reducing the number of likely explanations, we are slowly getting closer to understanding what causes these extreme events.
Nature, 2022. DOI: 10.1038/s41586-022-04841-8 (About DOIs).
