Ever since their discovery more than a decade ago, enigmatic flashes of radio waves have puzzled astronomers. These “fast radio bursts” (FRBs) pop up with startling frequency and intensity all across the sky, each emerging from unknown faraway extragalactic sources and packing the power output of up to hundreds of millions of suns into just a few fleeting milliseconds.
Now researchers are closing in on their origins.
A team studying one particular FRB some three billion light-years from Earth—known as FRB 121102, the only ever seen to repeat—has found it is engulfed by an extremely strong magnetic field. Such extreme magnetic fields have only previously been seen near neutron stars around the supermassive black hole at the center of our galaxy. The team suggests this FRB’s mysterious source is a very young and fast-spinning, highly magnetized neutron star—a magnetar—that may be orbiting a massive black hole. The findings are published in the January 11 Nature.
“For the first time, we’re getting some sense of the environment around the burst’s source—remote sensing from three billion light-years away!” says study co-author Shami Chatterjee, an astronomer at Cornell University. “We recognize this is piling one exotic thing atop another: We want an energetic magnetar without precedent, and we also want to put it next to a massive black hole. But we do have a similar example in our own galaxy. ”The magnetars near the Milky Way’s center, however, have yet to be seen emitting FRBs, which tend to come from much, much further away.
A curious property of FRBs confirms their vast distance from us—their radio waves have been “dispersed” by their passage through clouds of electrons that fill the space between stars and galaxies, smeared out in proportion to how far they have journeyed to reach Earth. That means FRBs could become best-in-class probes of cosmic structure, allowing researchers to determine not only the distance to any given FRB but also how much intervening material lies in interstellar and intergalactic space along its path. But to fully realize that revolutionary potential, astronomers must better understand what gives rise to FRBs in the first place, and whether the lone known repeating burster, FRB 121102, is a typical example or a fluke.
To learn more, the team periodically monitored the repeater across several months using two of the world’s largest radio telescopes, Arecibo Observatory in Puerto Rico and the Green Bank Telescope in West Virginia. FRB 121102 does not repeat with clockwork regularity; instead, its bursts are intermittent and, so far, impossible to precisely predict. The team ultimately captured and analyzed 16 bursts. The durations of each burst, which ranged from nine to 30 milliseconds, suggested the source is perhaps 10 kilometers across—the size of a typical neutron star.
Besides looking at the timing and dispersion of each radio burst, the researchers also measured its polarization—the way the burst’s light oscillated up or down, left or right, perpendicular to its direction of travel. When polarized radio waves pass through magnetic fields and charged particles, their polarization becomes twisted like a corkscrew—the denser the particles and the more intense the magnetic field, the greater the twisting. Polarization measurements showed FRB 121102’s twisting was enormous, rivaling the largest ever seen from an astrophysical source. The twisting was also rapidly changing, declining by 10 percent across a roughly half-year period. Whatever the source is, it must be a compact object surrounded by a dense, intensely magnetized cloud of plasma (a hot, ionized gas) moving at very high speeds.
“Can we maybe now understand how this extreme environment is related to the fact that this is the only known repeating FRB?” asks study co-author Jason Hessels, an astronomer at the University of Amsterdam and ASTRON Netherlands Institute for Radio Astronomy. “Maybe that extreme environment includes structures that can boost the brightnesses of the bursts like a magnifying lens.” Those structures could be particularly dense tendrils and knots of plasma whipping through the cloud surrounding the FRB source, so-called “plasma lenses” that would occasionally amplify ongoing radio emissions to help produce the repetitions. Without such a magnifying effect, repeated bursts are hard to explain—FRBs are so powerful that many models suggest their production would require the total physical destruction of their sources, such as the cataclysmic collision of two neutron stars.
The first inklings of FRB 121102’s true nature trace back to early 2017, when this so-far singular FRB was definitively linked to a region of intense star formation in a dwarf galaxy some three billion light-years from Earth. Such dwarfs are rich in pristine gas scarcely altered since the universe sprang into being during the big bang nearly 14 billion years ago—and that gas tends to form especially massive and short-lived stars, which end their lives by exploding with astonishing violence as “superluminous supernovae.” Those explosions, in turn, can leave behind particularly extreme remnants—stellar-mass black holes, for one thing, but also run-of-the-mill neutron stars and their supercharged kin, magnetars. What’s more, when astronomers zoomed in on the FRB’s locale within the dwarf galaxy, they saw something else nearby—a softer, steadier radio glow from a roiling cloud of plasma that could have been material ejected from a recent magnetar-forming supernova or burped out by a voraciously feeding black hole. At the time no one knew whether the FRB was actually associated with this cloud; this latest study all but confirms it lies embedded within.
“Last year’s localization was a game changer in a very direct way,” says Jim Cordes, a study co-author and astronomer at Cornell. “This latest result is more drilling into the FRB and its surroundings to tell us something about the environment surrounding what we call the ‘engine,’ the object producing these high-energy radio bursts.” That fearsome engine, Cordes and other co-authors say, is most likely a magnetar less than a century old—a relative newborn in comparison with those we know in the Milky Way, which are thought to have formed thousands of years ago. A magnetar so young should be spinning extremely fast, perhaps once per millisecond, but will rapidly lose rotational speed as its whirling magnetic field dumps immense amounts of energy into a surrounding shell of expanding plasma left over from the supernova that birthed it.
“As the magnetar spins down its magnetic field moves. And the field is so strong it takes the magnetar’s ironlike crust with it, cracking the crust to generate ‘starquakes’ and flares that drive energy like a piston out into the surrounding, dynamic nebula,” Cordes says. “That’s one possibility.” The other, he says, is a magnetar orbiting a massive black hole that is feeding on huge volumes of gas and dust. In that more general scenario the magnetar could periodically pass through debris disks and particle jets surrounding the black hole as it feeds, being bathed in material that is then ejected at high speed by the intense magnetic fields. In either case, the result could be a repeating FRB. If FRB 121102’s twisted polarization continues to unwind (following the 10 percent reduction across a half year already seen), that would suggest a surrounding nebula slowly expanding and dissipating, in support of the first scenario. If instead its surroundings continue displaying wild magnetic oscillations, that could be better evidence for something more like the black hole scenario.
Although these results go a long way toward solving the mystery of FRB 121102, Chatterjee says, they still remain frustratingly silent about the bigger questions: Do all FRBs come from one type of physical source? Do all FRBs repeat? “This is a ‘nature versus nurture’ problem,” he says. “Is it in the nature of FRBs that they all originate in this sort of extreme environment—or is this more of a nurture situation, where this one repeats because of its extreme environment, this strong magnetic field and plasma lensing? Both possibilities remain tantalizingly open.”
More answers should come soon, via new wide-field radio telescopes now coming online that should excel at detecting more FRBs, pinpointing their cosmic origins and charting their possible repetitions. One in particular, called CHIME (Canadian Hydrogen Intensity Mapping Experiment), is projected to detect anywhere between a few and a few dozen FRBs per day when it begins operations later this year, giving astronomers fresh hopes for peering deeper than ever before into the mysterious hearts of FRBs across the universe.