A galactic fast radio burst finally reveals its origin

Magnetars are spinning neutron stars, each left over from the explosion of a star tens of times the mass of the Sun, and they have magnetic fields 100 trillion times stronger than Earth’s. Strain induced by the intense magnetic field increases until it’s abruptly relieved in a starquake, which gives rise to characteristic bursts of x rays and gamma rays, depicted in figure 1.

Of the 30 magnetars currently known in our galaxy and the Magellanic Clouds, five have exhibited faint, transient radio pulses coincident with what is presumed to be the magnetar’s spin period. A leading model for repeating FRBs suggests that they come from extragalactic magnetars. However, for that model to hold, some magnetars must be capable of generating radio emissions that exceed those of local, galactic ones by orders of magnitude.

On 27 April 2020, the Burst Alert Telescope aboard NASA’s Neil Gehrels Swift Observatory detected multiple bursts of gamma rays and x rays coming from the magnetar SGR (soft gamma repeater) 1935+2154, which indicated heightened activity. The Fermi Gamma-Ray Space Telescope also reported multiple gamma-ray bursts from the same object.

By the next day, that region of the sky came into view of ground-based telescopes in the Western Hemisphere. The Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope in Penticton, British Columbia, detected an FRB coming from the direction of the signal. CHIME researchers posted a notice to the astronomy community reporting the detection of an FRB, dubbed FRB 200428, and included the data shown in figure 2a. The signal was well outside CHIME’s field of view and was only detected because of its extreme brightness.¹ 

Christopher Bochenek, a graduate student at Caltech, saw CHIME’s notice when he began his daily inspection of data collected by the radio telescopes that make up the university’s Survey for Transient Astronomical Radio Emission 2 (STARE2). “I saw the same burst in the STARE2 data, and I was so surprised I froze for a bit!” he says. The three STARE2 telescopes, in Utah and California, were designed explicitly to find an FRB in the Milky Way, which was expected to be rare but incredibly bright.² Bochenek confirmed that the signal was consistent with the one reported by CHIME. It was three orders of magnitude brighter than any observed radio pulse from known galactic magnetars.

Because of the Swift telescope’s gamma-ray alert, several space telescopes had been keeping a close eye on the same location. Four of them reported an x-ray burst that simultaneously occurred with the radio bursts.

The Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST) in China had also been observing SGR 1935+2154 in the preceding weeks, but the narrow-field telescope was not pointed toward the magnetar when CHIME and STARE2 reported their new finding. FAST did, however, report a lack of FRB-like events alongside 29 other x-ray bursts from SGR 1935+2154 in the days before FRB 200428 was announced.³ 

Bing Zhang of the University of Nevada, Las Vegas, explains, “The nondetection is significant because it indicates that an FRB’s association with a gamma-ray repeater is unique and rare.” That rarity could be because specific physical conditions required for FRB emission are difficult to satisfy or because FRBs have narrow beams that seldom point toward Earth. Two days after CHIME and STARE2 reported FRB 200428, FAST saw, from precisely the same location, another, weaker radio burst typical of a magnetar.

The observation of gamma-ray and x-ray emissions concurrently with an FRB allowed astronomers to make the first compelling link between FRBs and magnetars. Amanda Weltman of University of Cape Town says, “This is the first galactic FRB, our first FRB localized to an actual source and not just a host galaxy, and the first sign that a magnetar can produce bright enough bursts to be observed as FRBs.”

The discovery of FRB 200428 implies that active magnetars can produce FRBs that are bright enough to be detectable at extragalactic distances. Although the observed signal, when scaled for distance, was weaker than other extragalactic FRBs by a factor, the researchers estimated that if the signal had come from the location of other known FRBs, it would nonetheless be detectable. Figure 3 shows how the FRB’s energy and duration compare with radio emissions from other known astronomical objects. Those comparisons led the CHIME, STARE2, and FAST researchers to conclude that magnetars like SGR 1935+2154 could indeed be a dominant source of FRBs.

How, exactly, a magnetar produces FRBs is still up for debate. One proposed mechanism for the link is that a starquake causes a magnetar to generate short-lived flares of electrons and other charged particles that collide with those emitted during previous flares, thus creating a shock front and huge magnetic fields.⁴ Electrons swirling around the magnetic field lines emit bursts of radio waves, and the heated electrons emit x rays. Another possibility is that the starquake triggers disturbances in magnetic field lines near the magnetar’s surface. Those disturbances induce relativistic particles to stream from the magnetosphere and generate radio emissions.⁵ 

Bochenek says, “We knew the source would need to be a compact object and have strong magnetic fields and that magnetars and neutron stars can create coherent radio emissions. But it’s not like anyone expected magnetars to make such bright radio emission before the discovery of FRBs.” Further study could help astronomers home in on the specific mechanisms and circumstances that drive magnetars to generate FRBs and, more generally, determine how coherent radio emission can be made under extreme conditions.

More observations are needed to fully understand the physics behind FRBs. Investigations should include searching nearby galaxies for similar events and seeking nonradio counterparts. For example, a coincident neutrino burst detected from the same locale as an FRB could provide evidence of another magnetar-driven FRB mechanism that involves an ultrarelativistic shock wave interacting with thermal synchrotron photons. So far, researchers combing through data from neutrino observatories have not found a corresponding signal.

From a theoretical point of view, determining the specific signatures of different physical mechanisms from distinct astrophysical objects will be vital to future FRB research. Zhang says, “Before the discovery, people had been talking about more than 50 possibilities of producing FRBs. I believe that a lot of models are no longer competitive.” Still, there are likely multiple ways that repeating FRBs are created. It’s also possible that rare, one-off FRB events could arise from catastrophic events such as merging neutron stars.

The FRB source in the Milky Way may also answer open-ended questions, such as how frequently the signals repeat over years to decades, how distant and energetic bursts are, and whether similar pulses can be used to identify specific objects in other galaxies. West Virginia University’s Duncan Lorimer, who reported the first-ever detected FRB in 2007, points out that there has not yet been any evidence for rotational periodicity in repeating FRBs—as might be expected for a spinning source. He says, “I think ultimately FRBs could be, and are, produced by different sources.”