In early 2020, astronomers working with Canada's CHIME radio telescope reported something no one had seen before: a fast radio burst source whose activity follows a regular cycle. The source, FRB 180916.J0158+65, switches on and off roughly every 16.35 days. All of the 38 bursts recorded between September 2018 and February 2020 arrived inside a window of about five days in each cycle, and half of them fell within a span of less than a day. Outside that window, the telescope heard nothing.
That does not mean the source fires like a clock, sending one pulse every 16 days. It means there is a recurring active season, and within it the bursts still come at irregular times. The difference matters, and it is the key to why the discovery was so interesting.
What fast radio bursts are
Fast radio bursts, or FRBs, are flashes of radio waves that last from a fraction of a millisecond to a few milliseconds. The first was found in 2007, when Duncan Lorimer and his student David Narkevic combed through archived data from Australia's Parkes radio telescope recorded in 2001. It became known as the Lorimer burst.
They are astonishingly energetic. A typical burst is estimated to release, in about a millisecond, as much energy as the Sun puts out in roughly three days. Yet by the time the signal reaches Earth from millions or billions of light-years away, it is extremely faint, which is why sensitive radio dishes are needed to catch one.
Most FRBs have been seen only once. That makes them hard to study, because by the time anyone points a telescope at the spot, the burst is long gone. A minority repeat, and those are the ones astronomers can return to, measure in detail and trace back to a home galaxy.
How CHIME found the 16-day cycle
CHIME, the Canadian Hydrogen Intensity Mapping Experiment, sits in British Columbia. It has no moving parts. Instead, its long half-pipe reflectors watch a wide strip of sky as the Earth turns, which makes it superb at catching brief, unpredictable flashes. Its dedicated FRB project began reporting discoveries in large numbers from 2018.
In 2019 the collaboration announced a batch of eight new repeating sources, and FRB 180916.J0158+65 was among them. At first it did not stand out. But because CHIME looks at the same patch of sky every day, the team built up a long record of when the source was active and when it was quiet. Over more than a year, a pattern emerged. Bursts only ever turned up in the same phase of a cycle lasting about 16.35 days, give or take 0.15 days.
The results were posted as a preprint in January 2020 and published in Nature later that year under the title "Periodic activity from a fast radio burst source." The authors were careful to note that the true period could be a shorter alias of 16.35 days, a common uncertainty with observations that come once a day.
Where the signal comes from
FRB 180916 is one of the few bursts that has been pinned to a specific galaxy. High-resolution follow-up observations, announced in January 2020, located it in a spiral galaxy about 500 million light-years away. At the time, that made it the closest FRB host known. The source sits in a region where new stars are forming, which hints that whatever produces the bursts is relatively young.
That location already narrows the options. A supermassive black hole at a galaxy's centre is unlikely, because the source lies away from the core. Something star-sized is a better fit.
Why a cycle could happen
In astronomy, regular rhythms usually come from orbits or rotation. The CHIME team and others proposed several ideas, none of which has been confirmed:
- A binary system. The burst source might orbit a companion star every 16 days. For part of each orbit, the companion's wind of gas could block or bend the radio waves, so we only see bursts during the rest of the orbit.
- A slowly precessing magnetar. Magnetars are neutron stars with ferociously strong magnetic fields. If one wobbles like a spinning top, its beam might sweep past Earth only during part of each wobble.
- A very slowly rotating neutron star. An isolated object turning once every 16 days could produce a similar pattern, though such slow rotation would be unusual.
The magnetar idea gained weight a few months later. In April 2020, a burst with FRB-like properties was detected from SGR 1935+2154, a magnetar inside our own Milky Way. That showed magnetars can produce at least some fast radio bursts. It did not prove that every FRB comes from one, or explain FRB 180916's rhythm by itself.
Common misunderstandings
"It is a message from aliens"
A regular pattern can sound like a sign of intelligence, but natural processes produce rhythms all the time. The first pulsar, briefly nicknamed "LGM-1" for "little green men" after its discovery in 1967, turned out to be a spinning neutron star. No serious researcher has suggested FRB 180916's cycle is artificial, and the active window with random bursts inside it is exactly what physical models predict.
"It repeats exactly every 16 days"
Only the active window repeats. Within it, the number and timing of bursts vary. Some cycles produce several bursts and some produce none that CHIME can detect.
"A quiet period means the source stopped"
Silence in the data might mean the source is not emitting, or that its signal is blocked or too faint for the telescope at that time. Later studies at other radio frequencies have reported that the active window shifts slightly depending on frequency, with lower frequencies arriving later in the cycle. If that holds up, it is valuable because it can help separate competing explanations.
Why the discovery mattered
Before 2020, repeating FRBs looked chaotic. A predictable active window changed how astronomers could work. They could schedule other telescopes for the days when the source was likely to be on, rather than watching blindly for weeks. That made coordinated observations at many wavelengths far more practical.
It also raised a bigger question: do other repeaters have cycles we have not noticed yet? Some sources may simply be too faint, or observed too irregularly, for a pattern to show. FRB 121102, the first known repeater, was later reported to have a much longer activity cycle of around 157 days, though that result is less certain than the 16-day period of FRB 180916.
No single object has solved the FRB mystery, but this one turned a random-looking phenomenon into something that could be tested. Each new model now has to explain why the bursts arrive in seasons, why the seasons shift with frequency and why the source sits where it does.
Sources: CHIME/FRB Collaboration (arXiv:2001.10275; Nature, 2020): Periodic activity from a fast radio burst source; Wikipedia: Fast radio burst.
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