Fast radio bursts are exactly what their name suggests: a sudden wave of photons at radio frequencies that often last less than a second. Once the scientists had finished convincing themselves that they were not looking at equipment glitches, research was underway for what would produce the massive amounts of energy involved in a fast radio burst (FRB).
Discovery First FRB Repeater He told us that the process that generates the FRB does not destroy the object that produces it. In the end, the FRB associated with . was found Events at additional wavelengthsallowing to specify the source: magnetic star, a subset of neutron stars that contain the most extreme magnetic fields in the universe. While this is an excellent advance, it still doesn’t tell us anything about the physics of how a burst is produced – the knowledge that is supposed to tell us why most magnetars don’t produce it and why a burst tends to start and stop suddenly.
Now, researchers have identified a FRB that helps limit our ideas about what might produce it. The FRB itself appears to be a single event, but it consists of nine individual bursts approximately 215 milliseconds apart. The fast pace means that the source of the explosion must almost certainly be near the surface of the magnetar.
Bursts and sub-bursts
New work comes from Canada resonance instrument, which was established for other observations but has been shown to be sensitive to the many wavelengths that make up the FRB. CHIME scans a large area of the sky, which allows it to pick out FRBs despite the fact that they do not occur in the same place nearly twice.
An automated analysis pipeline that selects potential FRB events should have missed an event called FRB 20191221A, simply because it was much longer than FRBs as identified, taking approximately three seconds for radio emissions to ramp up and then descend back to levels background again. But the data was saved for future analysis because those three seconds seemed to contain several independent bursts, and it was these sub-bursts that prompted the system to flag the data.
While we identified recurring sources before, those produced single bursts with a long interval between them. By contrast, FRB 20191221A had an interval of only about 215 milliseconds between them.
In fact, the gaps between these sub-streams were remarkably regular. The researchers estimated the probability of discovering something that appears regular but isn’t actually regular as one in 10-11giving them a “high confidence” that the signal is cyclical.
Since that event, there has been no indication of another event from the same region as FRB 20191221A. It also appears to be from a source outside our galaxy.
close to the heart
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 kinds of extremes of energy needed for a FRB. But magnetars have intense 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 orbits of the atoms are distorted, which prevents the chemistry from happening anywhere near them.) So, it’s not clear how close the generation of FRBs is to the neutron star.
The timing of these sub-pulses strongly confirms that they are on the surface of the star. The millisecond-level separation of events corresponds to the spin speed of neutron stars that we see in many pulsars. So what we’re seeing with FRB 20191221A may be an extensive event on the surface of a neutron star that creates a beam that flashes across Earth as the star spins before fading out again. But given the length of the pulsars, the source must be much wider than any pulsar we’ve seen.
An alternative explanation could be that the star is spinning slowly, and we’re watching an event that shakes its crust, with the burst of emissions timing the crust’s vibration frequency. Once again, the extreme nature of neutron stars means that a “stellar earthquake” will have much more energy than we would see on Earth.
By contrast, it is difficult to understand how you can create this type of periodicity at a distance from the magnetar without a periodic source on the star itself.
All of this, however, is based on the assumption that FRB 20191221A represents FRBs in general. By looking through the CHIME data, the research team came up with two examples of what appears to be a similar frequency but fewer sub-brushes. The statistical certainty about whether the regular separation between them is, however, is less due in part to the smaller number of iterations.
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 narrowing down the number of possible explanations, we are slowly getting closer to understanding what results from these extreme events.
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