With the new radio observations, astronomers have ruled out the main explanation for the cyclical nature of a particularly interesting repetitive cosmic signal.
We are talking about the FRB 20180916B signal, which repeats at intervals of 16, 35 days. According to current hypotheses, this may be the result of interactions between closely rotating stars; however, the new detections - which include observations of fast radio bursts (FRBs) at the lowest frequencies - do not make sense for such a binary system.
“Strong stellar winds from the fast radio burst source satellite were expected to allow most of the blue, shortwave radio emission to leave the system. But redder, longwave radio should be blocked more or even completely,” says astrophysicist Ines Pastor-Marazuela of the University of Amsterdam and ASTRON in the Netherlands.
“Existing binary wind models predicted that the bursts should only glow blue, or at least last much longer there. But we saw two days of more blue radio bursts, and then three days of redder radio bursts. Now we rule out the original models - there should be something else".
Fast radio bursts are one of the most exciting mysteries of space. These are extremely short bursts of very powerful radio waves of short length - only a millisecond in duration, during which time as much energy is released as 500 million Suns. Most of the discovered FRB sources have only been seen once, making them unpredictable and difficult to study.
Several FRB sources have been rediscovered, although most of them did so on an irregular basis. FRB 20180916B is one of two exceptions that repeat itself in a loop, making it a great example for exploring these mysterious events.
Last year, scientists also received important information about what might be causing FRB - the first such signal ever found inside the Milky Way. It was ejected by a magnetar, a type of neutron star with an insanely powerful magnetic field.
But this does not mean that the case is completely resolved. We do not know why some FRBs repeat and others do not, for example, and why periodicity has been found only on rare occasions in repeating FRBs.
When FRB 20180916B was found to recur in cycles, 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, then the lower frequency and longer radio waves would have to change under the influence of the charged wind of particles surrounding the binary system.
Pastor-Marazuela and her colleagues used two telescopes for simultaneous FRB observations - the Low Frequency Array (LOFAR) radio telescope and the Westerbork Synthesizing Radio Telescope, located in the Netherlands. When they analyzed the data, they found redder wavelengths in the LOFAR data - meaning that binary winds could not have been present to block them.
As well as other low frequency absorbing or scattering mechanisms such as dense electron clouds.
"The fact that some fast radio bursts live in a clean environment relatively unclouded by 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 the elusive baryonic matter that remains unaccounted for in the universe."
So if the binary explanation is out of the question, what could be causing the periodicity?
One explanation, proposed last year, has to do with a single object, such as a spinning magnetar or pulsar. This explanation was thought to be worse for the data than the binary wind of charged particles, since these objects have oscillating rotation that creates periodicity, and none of them is known to oscillate as slowly.
But after the binary wind was excluded from consideration, thanks to the observations of LOFAR and Westerbork, the slowly oscillating magnetar began to be considered again. This suggests that we still have a lot to learn about both magnetars and FRBs.
"An isolated, slow-spinning magnetar best explains the behavior we found," said Pastor-Marazuela.
"This is very similar to detective work - our observations have significantly narrowed down the range of possible patterns of fast radio bursts."
The study was published in the journal Nature.