Mysterious radio signal spotted in Milky Way for first time

Three new studies trace the burst to a bizarre "magnetic star"—and help solve a major astronomical puzzle.

By Michael Greshko
Published 5 Nov 2020, 09:25 GMT
In April, China's Five-hundred-meter Spherical Aperture Telescope (FAST) radio telescope helped probe the properties of the ...

In April, China's Five-hundred-meter Spherical Aperture Telescope (FAST) radio telescope helped probe the properties of the magnetar SGR 1935+2154, which spawned the first fast radio burst ever detected in the Milky Way.

Photograph by Bojun Wang, Jinchen Jiang With Post Processing By Qisheng Cui.

In a thousandth of a second on April 28, a powerful burst of radio waves washed over Earth, lighting up radio telescopes in North America. Now, astronomers have tracked down the source of this strange signal—and the results could reveal the long-sought cause of some of the most mysterious cosmic signals ever recorded.

In three studies published this week in the journal Nature, an international group of scientists identified the blast as a fast radio burst, an extremely intense flash of radio waves that lasts no more than a few milliseconds. Telescopes have picked up such bursts before, but always from outside our galaxy. Researchers have wondered for years what could cause these ephemeral but powerful blasts, with speculation ranging from exploding stars to alien technologies.

Now we know that at least one source is likely an exotic stellar object called a magnetar: a type of young neutron star left over after a large star explodes that has an extremely powerful magnetic field.

“When I looked at the data for the first time, I froze and was basically paralysed with excitement,” Caltech graduate student Christopher Bochenek, a lead author of one of the studies, said in a Monday press briefing.

Canada's CHIME telescope was the first to detect the radio burst on April 28, 2020.

Photograph by Andre Renard, Chime Collaboration

The new signal is the first fast radio burst pinned down to a specific source, providing a unique opportunity to finally study one of these cosmic flashes in detail. “This gives us a whole new handle on [fast radio burst] progenitor theories,” University of Cape Town astrophysicist Amanda Weltman, who wasn’t involved with the studies, says in an email.

Searching for bursts in the darkness

Discovered in 2007, fast radio bursts are extremely difficult to study because they’re over so quickly. In the early days, some scientists were skeptical that they were genuinely from space and not a misidentified signal from a source on Earth, such as microwave ovens.

By 2013, the discovery of four more bursts confirmed their cosmic origins—and deepened their mystery. Three years later, astronomers announced the discovery of a repeating source, which they were able to trace back to a galaxy more than 2.6 billion light-years from Earth. Now, astronomers have found more than a hundred fast radio bursts—roughly 20 of which are repeating.

Because the bursts are so short, and because their sources are so far away, astrophysicists have struggled to work out what sparks these intense radio blasts. But on April 27, two NASA space telescopes picked up x-ray and gamma-ray bursts emanating from a magnetar in the Milky Way called SGR 1935+2154. The next day, ground-based radio telescopes in the Western Hemisphere picked up a signal from the same object.

Canada’s CHIME telescope—which is made up of more than a thousand radio antennas arranged like massive metal halfpipes—was the first to detect the radio burst coming from the same patch of sky as the magnetar. CHIME staff immediately sent out an alert to astronomers around the world, encouraging them to turn their telescopes toward the object.

The signal also washed over STARE2, an array of low-tech radio telescopes—each made out of a metal pipe and two cake pans—spread out across California and Utah. Bochenek, who designed STARE2 and led its analysis, says that the radio burst was so intense, a 4G mobile phone’s receiver could have theoretically picked it up.

Seeing the x-ray and radio bursts in the same tiny patch of sky strongly tied the fast radio burst to the magnetar—the first such link between this mysterious type of signal and a specific celestial object.

Decoding the mysterious burst

The signal from the magnetar is the most energetic radio blast ever recorded within our galaxy. But compared to other fast radio bursts, this one was actually weak, with about a thousandth the energy of the typical burst from outside the Milky Way.

Researchers say that they expect weaker bursts to go off more often than stronger ones; we just can’t detect them if they’re too far away. Taken together, the new studies strongly suggest that at least some of the faraway radio bursts also hail from magnetars.

“I don’t think we can conclude that all fast radio bursts come from magnetars, but for sure, our models that have magnetars as the origins of fast radio bursts are very probable,” CHIME’s Daniele Michilli, a postdoctoral fellow at McGill University in Montreal, said in the press briefing. The brightest radio bursts, for example, may be produced by objects other than magnetars, Weltman adds.

The data can also help refine theories for how magnetars could produce fast radio bursts. In an accompanying review also published in Nature, astrophysicist Bing Zhang of the University of Nevada, Las Vegas, outlined the two most compelling scenarios. In one, flares of particles ejected from the magnetar’s surface collide at extreme speeds with surrounding debris, creating a hot, highly magnetized maelstrom that could give off both x-rays and radio waves. In the other, fast radio bursts form as the magnetar’s super-intense magnetic field lines get tangled and disconnected, releasing vast amounts of energy in the process.

However, Zhang and his colleagues also found that the conditions behind a fast radio burst are somewhat rare. Hours before the April 28 burst, the team was monitoring the magnetar with China’s FAST radio telescope, the largest single dish in the world. But FAST didn’t see any fast radio bursts come from the magnetar despite the object giving off 29 x-ray flashes as they were watching.

In the press briefing, Zhang pointed out that it’d make sense for magnetars to release fast radio bursts only infrequently. If every magnetar flare that gave off x-rays also spawned radio blasts, astronomers would expect to see a hundred to a thousand times more of them than they have spotted so far.

For Weltman, the future of the field is as bright as the radio bursts themselves. Global networks of telescopes are poised to catch more of these intense surges, both within and outside the Milky Way. And as blasts of radio waves periodically wash over Earth, scientists will use them to rule out many of the theories describing the provenance of these cosmic tantrums—a winnowing of ideas that Weltman calls “the real beauty of good, honest science!”

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