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

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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