Rapid radio burst repeats in a strictly defined pattern and scientists can’t figure out why

After conducting new radio observations, astronomers have ruled out a basic explanation for the cyclic nature of a particularly interesting repeating cosmic signal.

We are talking about the signal FRB 20180916B, which repeats with a periodicity of 16.35 days. Existing hypotheses suggest it could be the result of interactions between closely rotating stars; but new detections – which include observations of fast radio bursts (FRBs) at the lowest frequencies – don’t make sense for such a binary system.

“Strong stellar winds from the fast radio burst source companion were expected to allow most of the blue, short-wave radio emission to leave the system. But the redder, longer-wavelength radio should be blocked more or even completely,” said astrophysicist Ines Pastor-Marazuela of the University of Amsterdam and ASTRON in the Netherlands.

“Existing models of binary winds predicted that the bursts should only glow blue, or at least last there much longer. But we observed two days of more blue radio bursts and then three days of more red radio bursts. Now we’re ruling out the original models – something else must be going on.”

Fast radio bursts are one of the most fascinating mysteries of the cosmos. They are extremely short bursts of very powerful short-length radio waves – lasting only milliseconds, during which time as much energy is released as 500 million suns. Most detected FRB sources have only been observed once, making them unpredictable and difficult to study.

A few FRB sources have been rediscovered, although most have done so irregularly. FRB 20180916B is one of two exceptions that repeat cyclically, making it an excellent example to study these mysterious events.

Last year, scientists also obtained important information about what may be causing FRB, the first such signal detected inside the Milky Way. It was erupted by a magnetar, a type of neutron star with an insanely powerful magnetic field.

But this does not mean that the case is completely solved. We do not know why some FRBs recur and others do not, for example, and why periodicity has only rarely been found in recurring FRBs.

When FRB 20180916B was found to repeat cyclically, one of the leading explanations was that the neutron star that emitted the burst was in a binary system with an orbit of 16.35 days. If this were the case, the lower frequency and longer radio waves would have to be altered by the charged wind of particles surrounding the binary system.

Pastor-Marazuela and her colleagues used two telescopes to simultaneously observe FRB, the Low Frequency Array (LOFAR) and the Westerbork Synthesizing Radio Telescope, both located in the Netherlands. When they analyzed the data, they found redder wavelengths in the LOFAR data – meaning that binary winds could not be present to block them.

Neither could other low-frequency absorbing or scattering mechanisms, such as dense electron clouds.

“The fact that some fast radio bursts live in a pure environment, relatively unclouded by the dense electron fog in the host galaxy, is very interesting,” said astronomer Liam Connor of the University of Amsterdam and ASTRON.

“Such pure fast radio bursts will allow us to find elusive baryonic matter that remains unaccounted for in the universe.”

So, if a binary explanation is ruled out, what could be causing the periodicity?

One explanation proposed last year had to do with a single object, such as a rotating magnetar or pulsar. This explanation was thought to be worse for the data than a binary wind of charged particles, since these objects have an oscillating rotation that creates periodicity, and none of them are known to oscillate as slowly.

But after the binary wind was excluded from consideration, thanks to LOFAR and Westerbork observations, the slowly oscillating magnetar has come back under consideration. And this suggests that we still have much to learn about both magnetars and FRBs.

“An isolated, slowly rotating magnetar best explains the behavior we found,” Pastor-Marazuela said.

“It’s very much like detective work — our observations have significantly narrowed the range of possible models for fast radio bursts.”

The study was published in the journal Nature.

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