A Milky Way flash implicates magnetars as a source of fast radio bursts

High-energy event nearby could help explain mystery signals from distant galaxies

illustration of a magnetar

A bright radio burst generated by a magnetar (one illustrated) in our galaxy hints that similar objects are responsible for at least some of the fast radio bursts in other galaxies, which have puzzled astronomers for over a decade.

L. Calçada/ESO

Astronomers think they’ve spotted the first example of a superbright blast of radio waves, called a fast radio burst, originating within the Milky Way.

Dozens of these bursts have been sighted in other galaxies — all too far away to see the celestial engines that power them (SN: 2/7/20). But the outburst in our own galaxy, detected simultaneously by two radio arrays on April 28, was close enough to see that it was generated by a highly magnetic neutron star called a magnetar.

That observation is a smoking gun that magnetars are behind at least some of the extragalactic fast radio bursts, or FRBs, that have defied explanation for over a decade (SN: 7/25/14). Researchers describe the magnetar’s radio burst online at arXiv.org on May 20 and May 21.

“When I first heard about it, I thought, ‘No way. Too good to be true,’” says Ben Margalit, an astrophysicist at the University of California, Berkeley, who wasn’t involved in the observations. “Just, wow. It’s really an incredible discovery.”

In addition to giving magnetars an edge over other proposed explanations for FRBs, such as those involving black holes and stellar collisions, observations of this Milky Way magnetar may clear up a debate among theorists about how magnetars crank out such powerful radio waves.

Researchers first noted an intense radio outburst from a young, active magnetar about 30,000 light-years away, dubbed SGR 1935+2154, in an astronomer’s telegram. The Canadian Hydrogen Intensity Mapping Experiment, or CHIME, radio telescope in British Columbia had detected about 30 decillion, or 3 × 1034 ergs of energy from the burst. That was far brighter than any flash of radio waves previously seen from any of the five magnetars in and around the Milky Way known to emit radio pulses.

That report inspired another group of astronomers to check concurrent data from the Survey for Transient Astronomical Radio Emission 2, or STARE2, detectors in the southwestern United States. STARE2, which watches the sky for radio signals at a different set of frequencies than CHIME, measured a whopping 2.2 × 1035 ergs from the burst.

“This thing put out, in a millisecond, as much energy as the sun puts out in 100 seconds,” says Caltech astronomer Vikram Ravi, who was on the team that analyzed the STARE2 data. That made this event 4,000 times as energetic as the brightest millisecond radio pulse ever seen in the Milky Way. If such an intense burst had happened in a nearby galaxy, it would have looked just like a fast radio burst.

“I was basically in shock,” says radio astronomer Christopher Bochenek of Caltech, who combed through the STARE2 data to find the burst. “It took me a while, and a call to a friend, to calm me down enough to go and make sure that this thing was actually real.”

The weakest FRB that has been observed in another galaxy was still about 40 times more energetic than SGR 1935+2154’s radio flare. But that’s “pretty close, on astronomical terms,” says Keith Bannister, a radio astronomer at Australia’s Commonwealth Scientific and Industrial Research Organization in Sydney, who was not involved in the work. Magnetars like this “could be responsible for some fraction, if not all of the FRBs that we’ve seen so far,” he says. “This motivates future studies to try and find similar sorts of objects in other, nearby galaxies.”

If magnetars do generate extragalactic FRBs, then SGR 1935+2154 could give new insight about how these objects do it. Theorists currently have many competing ideas about magnetar FRBs, Margalit says. Some think the FRB radio waves originate right in the thick of the star’s intense magnetic fields. Others suspect radio waves are emitted when matter ejected from the magnetar collides with material farther out in space.

Different magnetar FRB scenarios come with different predictions about the appearance of X-rays that should be emitted along with the radio waves. Extragalactic FRBs are so far away that “the X-rays are kind of hopeless to detect,” Margalit says. But SGR 1935+2154 is close enough that spaceborne detectors saw a gush of X-rays from the magnetar at the same time as the radio burst. A closer look at the brightness, timing and frequency of those X-rays could help theorists evaluate magnetar FRB models, Margalit says.

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