Enlarge / An event on the surface of a magnet can produce rapid bursts of radio.
Rapid bursts of radio are exactly what their name implies: a sudden rise of photons at radio frequencies that often lasts less than a second. Once the scientists had finished convincing themselves that they were not looking at the team’s mistakes, the search began for what was producing the large amounts of energy involved in a fast radio (FRB).
The discovery of the first repetitive FRB told us that the process that generates an FRB does not destroy the object that makes the production. Finally, an FRB was found that was associated with events at additional wavelengths, which made it possible to identify the source: a magnet, a subset of neutron stars that has the most extreme magnetic fields in the universe. While this represents excellent progress, it still tells us nothing about the physics of how the explosion occurs, a knowledge that would presumably tell us why most magnets do not produce them and why the explosion tends to start and stop so suddenly.
Now, researchers have identified an FRB that helps limit our ideas about what can produce them. The FRB itself appears to be a single event, but consists of nine individual bursts separated by about 215 milliseconds. The fast pace means that the source of the explosion should almost certainly be close to the surface of the magnet.
Explosions and sub-explosions
The new work comes from Canada’s CHIME instrument, which was created for other observations, but turns out to be sensitive to many of the wavelengths that make up an FRB. CHIME scans a large area of the sky, allowing you to choose FRB even though they almost never happen twice in the same place.
Announcements
The automated scanning channel that selects possible FRB events should have missed an event named FRB 20191221A, simply because it was much longer than FRBs as defined, taking almost three seconds for radio emissions to increase and then walk down. back to background levels. But the data was saved for future analysis because it appears that these three seconds contain several independent bursts, and these sub-bursts are what have activated the system to mark the data.
The individual bursts of this event are visible in a wide range of wavelengths.
Although we have previously identified repetitive sources, those produced individual bursts with a long separation between them. FRB 20191221A, by contrast, had a separation of only about 215 milliseconds between them.
In fact, the gaps between these sub-bursts were remarkably regular. The researchers estimated the likelihood of detecting something that looks as regular without actually being regular as one in 10-11, which gave them a “high confidence” that the signal is periodic.
Since that event, there is no indication of another event in the same region as FRB 20191221A. It also appears to be from a source outside our galaxy.
Near the core
But it is 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 energies needed for an FRB. But magnets have extreme magnetic fields that extend the high-energy environment far beyond the surface of the neutron star. (The strength of their fields is so strong that the normal orbitals of the atoms are distorted, preventing chemistry from occurring near them.) Therefore, it is not obvious at what distance from the neutron star FRBs are generated. .
Announcements
The timing of these sub-bursts strongly argues that it is on the surface of the star. The millisecond level separation between events is consistent with the rotational speed of neutron stars we see in many pulsars. Thus, what we are seeing with FRB 20191221A could be a broad event on the surface of the neutron star that creates a beam that flashes across the Earth with the rotation of the star before it fades. Given the length of the pulses, however, the source should have been much wider than any pulsar we have observed.
An alternative explanation could be that the star rotates slowly and we are seeing an event that has made its crust vibrate, with the explosion of emissions timed to the vibration frequency of the crust. Again, the extreme nature of neutron stars means that an “earthquake” would have much more energy than we would have ever seen on Earth.
On the contrary, it is difficult to understand how this type of periodicity can be generated at a distance from the magnet without having a periodic source in the star itself.
All this, however, is based on the assumption that FRB 20191221A is representative of FRBs in general. When searching for CHIME data, the research team found two examples of what appears to be a similar periodicity but with a smaller number of sub-bursts. However, partly due to the lower number of repetitions, statistical certainty about 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 last decade. By gradually reducing the number of probable explanations, we are slowly coming closer to understanding what produces these extreme events.
Nature, 2022. DOI: 10.1038 / s41586-022-04841-8 (About DOI).