After making new observations by radio, astronomers have ruled out a first-order explanation for the cyclical nature of a particularly curious repetitive space signal.
The signal in question is FRB 20180916B, which is repeated at a frequency of 16.35 days. According to existing models, this could result from interactions between stars in narrow orbit; however, new detections – which include rapid burst (FRB) observations at lower frequencies – do not make sense for such a binary system.
“The strong stellar winds of the fast radio partner were expected to let most of the blue light and short wavelength escape from the system. But the redder long wavelength radio would have to block more or even completely, ”said astrophysicist Inés 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. But we saw two days of bluer radio bursts, followed by three days of redder bursts. Let’s discard now the original models – something else must happen “.
Radio bursts are one of the most fascinating mysteries of the cosmos. They are extremely short bursts of very powerful short wavelength radio waves, such as, lasting only milliseconds, and discharge as much energy as 500 million suns in that time. Most of the FRB sources we have detected have only been seen once; this makes them unpredictable and difficult to study.
Some recurring FRB sources have been detected, although most have done so irregularly. FRB 20180916B is one of two exceptions that are repeated in cycle, which makes it a great case to learn more about these mysterious events.
Last year we also gained a major advantage over what FRBs might be causing: the first signal of this type detected from the Milky Way. It was spit out by a magnet, a type of neutron star with an incredibly powerful magnetic field.
But that doesn’t mean the case is completely resolved. We do not know why some FRBs are repeated, and others not, for example, and why, for FRBs that are repeated, periodicity has only been detected rarely.
When it was verified that the FRB 20180916B was repeated in a cycle, one of the main explanations was that the neutron star emitting the explosion was in a binary system with an orbit of 16.35 days. If this were the case, then the lower and lower frequency radio wavelengths would have to be altered by the wind laden with particles surrounding the track.
Pastor-Marazuela and colleagues used two telescopes to make simultaneous observations of the FRB: the Low Frequency Array Radio Telescope (LOFAR) and the Westerbork Synthesis Radio Telescope, both based 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.
However, there could be no other low-frequency absorption or scattering mechanisms, such as dense electron clouds.
“The fact that some fast bursts live in clean environments, relatively hidden by any dense electron fog in the host galaxy, is very exciting,” said astronomer Liam Connor of the University of Amsterdam and ASTRON.
“These fast, naked bursts will allow us to hunt down the elusive baryonic matter that is not yet in the universe.”
So if the binary explanation is ruled out, what could be causing the periodicity? Well, they’re not aliens yet, sorry.
An explanation suggested last year involves a single object, such as a magnet or rotating pulsar. This was thought to be a more deficient fit for the data than the binary wind of the charged particles, as these objects have an oscillating rotation that produces periodicity and nothing is known to oscillate slowly.
But with the binary wind off the table, thanks to the observations of LOFAR and Westerbork, a slowly oscillating magnet reappears. And that suggests we still have enough to learn about both magnets and FRB.
“An isolated, slowly rotating magnet explains better the behavior we discovered,” Pastor-Marazuela said.
“He feels a lot like a detective: our observations have greatly reduced what fast radio models can work.”
The research has been published in Nature.