Rapid radio bursts are exactly what their name implies: a sudden burst of photons at radio frequencies that often last less than a second. Once the scientists had finished assuring themselves that they were not observing the instrument’s disturbances, the search continued as to what was producing the large amounts of energy involved in a rapid radio burst (FRB).
The discovery of the first repeatable FRB told us that the process that produces an FRB does not destroy the thing it produces. Eventually, an FRB was found that was associated with events of additional wavelengths, allowing the source to be identified: a magnetar, a subset of neutron stars that have some of the universe’s most extreme magnetic fields. While this represents excellent progress, it still tells us nothing about the physics of how explosions arise—knowledge that will likely tell us why most magnets don’t produce them and why explosions start and stop abruptly. it happens.
Now, researchers have identified an FRB that helps narrow our thoughts about what might be causing 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 speed means that the source of the explosion must almost certainly be near the surface of the magnetar.
Eruption and sub-explosion
The new work stems from Canada’s CHIME instrument, which was built for other observations but tends to be sensitive to the many wavelengths that make up the FRB. CHIME scans a vast area of sky, allowing FRBs to be picked out despite the fact that they are almost never in the same location twice.
The automated analysis pipeline that picks up on potential FRB events should have missed an event called FRB 20191221A, simply because it was much longer than the FRB, as they are defined, to ramp up radio emissions and then back up. It takes about three seconds to go down. again at the background level. But the data was saved for future analysis because it appears that several independent bursts occur in three seconds, and it is these sub-bursts that trigger the system to flag the data.
Whereas we have previously identified repetitive sources that generate single eruptions with long separations between them. FRB 20191221A, in contrast, had a difference of only about 215 milliseconds between them.
In fact, the intervals between these sub-eruptions were remarkably regular. Researchers estimated that the odds of detecting something that looks regular without actually seeing it as regular as one in 10-1 1Giving them “high confidence” that the signal is periodic.
Since that event, there has been no sign of another event from the same area as FRB 20191221A. It also appears from a source outside our galaxy.
close to core
But it is actually the periodicity that tells us something about the nature of the FRB. Neutron stars themselves are very extreme environments, so their surfaces can produce the immense energy required for an FRB. But magnetars have extreme magnetic fields that extend the high-energy atmosphere beyond the surface of the neutron star. (The strength of their fields is so strong that the normal orbits of the atoms are distorted, preventing chemistry anywhere near them.) Therefore, it is unclear how close neutron star FRBs arise.
The timing of these sub-blasts strongly argues that it is at the surface of the star. The millisecond-level separation between events is consistent with the rotational speed of neutron stars that we observe on many pulsars. So what we’re seeing with FRB 20191221A may be a massive event on the surface of the neutron star that creates a beam that twinkles at Earth and twinkles with the star’s rotation before exiting back. Given the length of the pulses, however, the source must have been much wider than any pulsar we’ve ever seen.
An alternative explanation could be that the star is slowly spinning, and we are seeing a phenomenon that has set its crust vibrations, with bursts of emission at the time of the crust’s vibrational frequency. Again, the extreme nature of neutron stars means that a “starquake” would have far more energy than what we would ever see on Earth.
Conversely, it’s hard to understand how you can produce this kind of periodicity at a distance from the magnet without a periodic source on the star.
However, this is all based on the assumption that FRB 20191221A is generally representative of FRB. By exploring the CHIME data, the research team has come up with two examples that appear to have a similar periodicity but with a smaller number of sub-eruptions. Partly because of the small number of repeats, however, statistical certainty about whether they have regular segregation is very low.
So, while there is still some uncertainty as to how representative FRB 20191221A is, it is the kind of progress that has slowly brought us closer to understanding FRBs over the past decade. By gradually reducing the number of possible explanations, we are slowly getting closer to understanding what causes these extreme events.
Nature, 2022. DOI: 10.1038/s 41586-022-04841-8 (About DOI).