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For about four days, the radio waves would arrive at random. Then, for the next 12, nothing.

Then, another four days of haphazard pulses. Followed by another 12 days of silence.

The pattern—the well-defined swings from frenzy to stillness and back again—persisted like clockwork for more than a year.

Dongzi Li, a doctoral student at the University of Toronto, started tracking these signals in 2019. She works on a Canadian-led project, CHIME, that studies astrophysical phenomena called “fast radio bursts.” These invisible flashes, known as FRBs for short, reach Earth from all directions in space. They show up without warning and flash for a few milliseconds, matching the radiance of entire galaxies.

Astronomers don’t know what makes them, only that they can travel for millions, even billions, of years from their sources before reaching us. In the past decade, astronomers managed to detect about 100 of them before they vanished.

Li was monitoring FRBs, tracking their arrival times at a radio telescope in British Columbia, when she noticed that unusual pattern from one FRB source—four days on, 12 days off. (This is, perhaps, the purest definition of radio silence.)

The FRB, known by the bar-code-esque designation 180916.J0158+65, is the first to show this kind of regular cadence. Astronomers traced the source to a spiral galaxy about 500 million light-years away, where it’s still going strong.

The paper on this discovery, published earlier this month, marked the end of formal observations in February. Like so many people this year, Li has spent most of her days at home, rarely venturing beyond the walls of her small apartment in Bonn, Germany, but the Canadian observatory continues to scan the skies, catching the fleeting FRBs as little smudges of black against a plot of white noise. When Li and I spoke this week, she told me she’s still checking—and the rhythm is still there.

The discovery is an intriguing addition to a growing inventory of knowledge in a field whose earliest evidence was almost dismissed as a fluke. The first FRB was discovered in 2007, buried deep in archival data of a telescope in Australia, while astronomers were looking for another astrophysical phenomenon. The signal was thought to be a telescope artifact, a trick of light masquerading as a cosmic curiosity. And then similar signals started showing up in observations at other telescopes.

Astronomers accepted that they had detected a real event, but they still thought FRBs were one-offs. The flashes were so intense, even after crossing unfathomable distances in space, that whatever had produced them seemed unlikely to survive the cataclysm. But then astronomers found a repeater, a source of FRBs capable of erupting again and again, sometimes several times in less than a minute.

When astronomers managed to trace an FRB to its home galaxy for the first time, they found a small, lively galaxy, where new stars blinked into existence more than 100 times faster than in our own Milky Way. So FRBs must come from these kinds of environments, they thought. But then astronomers found that some FRBs originated in larger, mellower galaxies too.

“It seems like every time the scientific community converges on a possibility of what FRBs might be, some other observation happens that throws all these speculations out the window,” Kaitlyn Shin, an astrophysics graduate student at MIT who worked on the discovery of the pattern-bearing FRB, told me. “Now all the other theories going forward have to find a way to account for this periodicity.”

And not just from the FRB that Shin and Li’s team found, either; a different team reported this month the discovery of another signal that pulses in a much longer pattern—a 157-day cycle, with 90 days of bursts, followed by 67 days of silence. Many other FRB sources might also follow distinct rhythms, but telescopes just haven’t observed them long enough to spot the tempo.

The nature of the objects that produce FRBs remains a mystery, but astronomers are collecting clues. The most important one to date appeared just two months ago—2020 has been a great year for FRBs, truly—when the observatory Li works with detected an FRB-like event inside our very own galaxy. The flash came from an astrophysical object called a magnetar, an ultramagnetic type of neutron star, the leftover core of an aging star.

For once, FRB astronomers weren’t entirely shocked at a discovery in their field. Magnetars currently top the list of theories for the engines of these mysterious bursts.

Astronomers have now come up with a few potential explanations for the source of the FRB that jams to its own distinct tune. Maybe the object is spinning and wobbling in such a way that its light points toward Earth only every four out of 16 days, which, from our perspective, would look like periodic bursts. Maybe it’s actually two objects—a neutron star orbiting another neutron star or even a black hole—locked in an orbit that squishes one star so much that it flares as it swings around. Maybe the source resides near a cloud of interstellar gas that amplifies its radio emissions, like a cosmic magnifying glass, as it passes through.

With so many scenarios on the table, I couldn’t resist asking astronomers about the option at the very edge of possibility, unlikely but also impossible to rule out: aliens. I admit that, despite knowing better, when I learned that astronomers had detected a distinct pattern emanating from outside the solar system, my mind jumped to Contact, the ’90s classic starring Jodie Foster as Ellie Arroway, a scientist obsessed with extraterrestrial life. When Li told her colleagues about the signals she saw, did they sprint from console to console in an operating room like Arroway did, scrambling to turn up the signal louder, clearer?

No, because the story with FRBs—the story with most mystifying astrophysical phenomena—is that it’s never aliens. Although, people more qualified than I am are also considering that, okay, maybe, these might, on the off chance, be alien signals: Avi Loeb, the Harvard astrophysicist known for entertaining ET explanations, this week drew a connection between Li’s FRB and a planet in the habitable zone of Proxima Centauri, the star closest to our sun. The planet takes 16 days to orbit its star, the same period observed in the FRB’s behavior, and Loeb suggested that perhaps the radio waves come from that planet, whose inhabitants have figured out how to harness and beam starlight, when their world turns our way.

But although the newly found FRB is indeed weird, it’s probably not a beacon from an advanced civilization. “This shares a lot of properties with other sorts of FRBs, which are not regular at all, so we don’t have any reason to believe that this one in particular is special,” Vikram Ravi, an astronomy professor who wasn’t involved in the research but who has discovered several FRBs, told me.

On top of that, Ravi expects a bit more from any aliens trying to send intergalactic hellos. “The signals are quite broadband, whereas it’s much more efficient to communicate in narrowband,” he said. “One would hope that if someone was communicating, it would be a bit more well defined.”

FRBs are beacons of another kind, and astronomers have detected more than is widely known. In January, members of CHIME wrote in a paper posted on the preprint repository arXiv.org that they have detected 700 FRBs in less than a year. The catalog, when it is formally published, will increase the number of known signals sevenfold. As these radio waves propagate through space, they pass through all kinds of matter, from the most luminous galaxies to nearly invisible wisps of cosmic dust, slowing down here and there. These encounters are encoded in the radio waves, and scientists can pick them out when the signals reach us. Astronomers are particularly interested in studying the wispy material that lies between galaxies, because “we have no information other than what FRBs are beginning to give us,” says Shami Chatterjee, an astrophysicist at Cornell University who studies FRBs and was not involved in the new research. To see inside this expanse, astronomers need the help of these strange signals that they still barely understand.

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