The strangest fast radio burst (FRB) yet is helping us to narrow down the possibilities of what causes these odd, powerful blasts of radio waves from space. The unusual patterns we see in its light suggest that it may be coming from a wobbly neutron star.
FRBs generally last only a few milliseconds, but some of them have been observed to repeat, bursting many times from the same location. We don鈥檛 know what causes them, although everything from enormous black holes to strange quark stars to alien spaceships have been put forward as explanations.
Sorting it out has been made harder because the timing of the repeating FRBs has seemed random, and many potential sources of repeating blasts should result in predictable patterns.
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In February, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) found such a pattern in a repeating FRB鈥檚 flashes for the first time. This FRB seems to have a 16-day cycle: all of the bursts arrive in four-day windows, followed by about 12 days without bursts and then another window of activity.
That regularity gave us a clue in the search for the sources of FRBs, and several different research groups came to similar conclusions: these patterns could be caused by the wobble of a highly magnetised neutron star called a magnetar.
Magnetars emit powerful beams of light, which we could detect as FRBs when they pass over Earth. However, these stars spin so quickly that if the pattern of bursts was caused by a simple spinning magnetar, we would see a period of bursts every few seconds rather than over a period of weeks.
To find out if a magnetar could produce what we see, Yuri Levin at Columbia University in New York and his colleagues 鈥� as well as several other groups of researchers 鈥� evoked a wobble in its spin.
鈥淚f you throw a body into the air and set its initial spin around some random direction, if the body is not too symmetric you will observe it tumble,鈥� says Levin. Magnetars aren鈥檛 perfectly spherical because their fast spins and powerful magnetic fields deform their shape. As they rotate, that tumbling takes the form of a slight wobble, like a spinning top on an uneven surface.
The wobble could come from asymmetries in the magnetar itself, or it could be caused by gravitational effects of a companion orbiting it, as suggested by Huan Yang at the Perimeter Institute in Canada and his colleagues.
Either of these mechanisms would cause any beam of light emitted by the magnetar to trace a circle through the sky. If that circle takes 16 days to complete, the pattern matches what CHIME observed: four days in which we can see the beam, and then 12 days as it circles back around.
鈥淚 think all of these models make predictions that will be testable if not in the next month, within the year,鈥� says Levin. 鈥淭his is a spectacular source, and as long as it keeps providing bursts and doesn鈥檛 turn off for some reason, everything about it will be extremely well-measured.鈥�
If this FRB comes from a magnetar, maybe the rest of them do as well: magnetars that turn off and on could produce repeating FRBs that appear not to have any patterns in their signals, as well as FRBs that don鈥檛 appear to repeat, says Yang.
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