Strange behavior caught by two radio observatories may send theorists back to the drawing board.

Radio observatories (artist's concept)
Artist’s concept of the two radio observatories involved in a new fast radio burst study: dishes at the Westerbork Synthesis Radio Telescope (left) detected a periodic, short signal from a fast radio burst at high frequencies. But the low-frequency signal from the same source, caught the LOFAR telescope, appeared much later.
Joeri van Leeuwen

Fourteen years ago, the first fast radio burst (FRB) was discovered. By now, many hundreds of these energetic, millisecond-duration bursts from deep space have been detected (most of them by the CHIME radio observatory in British Columbia, Canada), but astronomers still struggle to explain their enigmatic properties. A new publication in this week’s Nature “adds a new piece to the puzzle,” says Victoria Kaspi (McGill University, Canada). “In this field of research, surprising twists are almost as common as new results.”

Most astronomers agree that FRBs are probably explosions on the surfaces of highly magnetized neutron stars (so-called magnetars). But it’s unclear why most FRBs appear to be one-off events, while others flare repeatedly. In some cases, these repeating bursts show signs of periodicity, and scientists had come up with an attractive model to explain this behavior, involving stellar winds in binary systems.

However, new observations by European radio telescopes may rule out this model.

Astronomers knew that FRB 20180916B, located in a galaxy some 460 million light-years away, produces multiple bursts about every 16 days, during a ‘window’ that lasts for a few days. “The idea was that the magnetar is part of a binary system with a 16.29-day period,” says Inés Pastor-Marazuela (University of Amsterdam and ASTRON, The Netherlands), the first author of the new paper. If the companion star had a thick stellar wind that absorbs radio waves, the bursts would only be visible when the magnetar was on ‘our’ side of the orbit, she explains.

However, simultaneous observations of FRB 20180916B by the Low-Frequency Array (LOFAR) and the 14-dish Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands challenge the predictions of this model. Since stellar winds should better absorb lower-frequency radio waves than higher-frequency ones, astronomers expected that the bursts observed by LOFAR (down to 110 megahertz) would only be visible in a narrower time window than the bursts observed by WSRT (at around 1.4 gigahertz). “We found the exact opposite,” says coauthor Joeri van Leeuwen (University of Amsterdam and ASTRON, The Netherlands). Moreover, the peak in the number of high-frequency bursts preceded the low-frequency peak by a few days, which also isn’t expected in the binary wind model.

“I agree that the observations are challenging for the model,” says Kaspi, who’s part of a team that has independently studied the LOFAR data (which are publicly available) but didn’t have access to the simultaneous Westerbork observations. However, she’s not yet convinced that the binary idea is completely ruled out. “We need more sources and better statistics.”

What could be an alternative explanation? Perhaps, says Pastor-Marazuela, the 16.29-day period is actually the rotation period of the burst source, instead of its orbital period. If the explosions originate in a small, localized region of the magnetar’s surface, this region will be carried in and out of sight by the compact object’s rotation.

Kaspi counters that a rotation period of 16.29 days would be incredibly long: Magnetars (and neutron stars in general) usually complete tens, hundreds, or even a few thousand revolutions per minute. “But nature can be very creative,” she adds. “Never say never.”

FRB 20180916B could be a very unusual case, says van Leeuwen. In particular, he is surprised by the fact that no single burst was detected by both LOFAR and WSRT, even though the two facilities were observing simultaneously. “It’s something I had never expected,” he says. But even if this particular source is special, it could shed more light on the properties of FRBs in general. “Think of Oliver Sacks, the famous neurologist,” van Leeuwen says. “He learned a lot about the human brain by studying his most interesting patients.”


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Peter Wilson

August 26, 2021 at 1:34 pm

Could the period of 16.29 days be its precession period?

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Andrew James

August 27, 2021 at 12:11 am

If I were to guess, I would say that the FRBs source was acting as a exotic capacitor and the discharge of radio waves by some parasitic effect or leakage. This would mean that what we are observing is a kind of electric dwarf nova, where the electrical energy stored is suddenly released.

It is unlikely to be a precessional effect, because that would imply the need or companion star or body, in which there is no evidence at all. Moreover, this would mean the object is continuously creating these radio waves, leading to issues in how the radiation could be created.

Also the comment "...may send theorists back to the drawing board." always looks to me like a carrot dangling out for the fringe novice scientists, who just look for cracks in scientific theories to highlight imperfections then exploited it for their own crackpot theories. Science is built on existing knowledge, and is explaining observation is hypothesised on that limited knowledge. It is a very rare circumstance to find phenomena based on totally new unknown form of science. e.g. FRBs must be made by an energetic source that creates the observed outburst by established scientific theory even though we don't know the circumstances in how it is generated. Sure we might have to go back one or two steps in our hypothesis but that does not mean, as implied in the headline, that we throw the baby out with the bathwater. I know you want to get people's attention to this story, but you might like to reconsider not. overdramatising things where the underlying basic science is not what is being questioned.

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August 28, 2021 at 11:05 pm

Don't know if precession is likely or not [not having actually read the following yet], but it is certainly plausible enough that there's already one paper up at examining 2 scenarios:

One of the mechanisms they look at, BTW, doesn't require a second compact body, although it seems improbable that it's happening in this case.

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Andrew James

August 29, 2021 at 9:23 pm

Question. What we need in regards this object is find is the radiation is being continuously created by the FRB or is it really periodic?

For precession to work, surface of the object would have to be fluid-like to create perturbations so the spin axis can precess. Else the only other scenario is having an external gravitational body.

If the source is a magnetar, then it's reasonable to assume that its own properties maybe directly related to the outbursts. We need more observations to understand the nature of periodicity - preferably from multiple sources. Thanks for the link.

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