Nobody noticed when an Australian radio telescope captured a fleeting explosion coming from far beyond the Milky Way in 2001. Records of the powerful flare sat unseen for more than half a decade until a group of scientists sifting through archival data spotted the eruption—a so-called fast radio burst (FRB). According to one of those scientists, astrophysicist Duncan Lorimer of West Virginia University, the burst produced as much energy in a few thousandths of a second as the sun does in a month.
Today researchers know that these explosions happen at least 800 times a day all over the sky, and they are one of the most active topics in astrophysics. Although much about FRBs remains unknown, in just the past year a clearer picture has started to emerge. “I think we're closer to understanding what some FRBs are,” says Ziggy Pleunis, an astrophysicist at the University of Toronto. “But as we've been going on this quest, new discoveries have led to new questions.”
The study of FRBs is now at an inflection point. A torrent of new detections and deeper studies have elevated certain models of their inner workings while eliminating others, and several upcoming projects should help further narrow the possibilities. Meanwhile scientists have learned that the bright light of an FRB carries within it a record of the contents of the intergalactic depths it traversed along its way to Earth, providing information about galaxies and the material between them that no other mechanism can.
In April 2020 three separate research teams detected an enormous blast of radio energy coming from a magnetar located in the Milky Way. Magnetars are an extreme kind of neutron star, a city-sized remnant left behind when a massive star dies in a supernova. A magnetar's magnetic field can be so strong that approaching within 1,000 kilometers of one would disrupt your body's atomic nuclei and electrons, causing you to effectively dissolve.
Magnetars were already a leading candidate for the source of FRBs. But the few dozen known magnetars in our galaxy had never been observed to produce eruptions that might resemble the phenomena. The discovery of a short and formidable radio burst from a galactic magnetar called SGR 1935+2154 was exactly what researchers had been missing. If the same burst had come from another galaxy, its signature would have been indistinguishable from a typical FRB. “That was a huge moment for the field,” says Kenzie Nimmo, an astronomer at the University of Amsterdam. “It alleviated all doubt that at least some FRBs come from magnetars.”
How, exactly, a magnetar can produce an FRB is still the subject of debate. Most theories involve some kind of jarring starquake or an explosion caused when a magnetar's twisting magnetic field lines snap and reconnect. Events such as these could directly generate an FRB's flash, or they might make a shock wave that heats up surrounding material, incinerating dust and turning gas into plasma to produce light as it travels outward.
Several telescopes saw an x-ray flash arriving just after SGR 1935+2154's radio signal, suggesting that whatever released the radio energy also generated more complicated side effects. Many details are still unclear. “Did this happen on the surface of the star, or in the magnetosphere, or in the material around the magnetar?” asks Emily Petroff, an astrophysicist also at the University of Amsterdam. “We still don't really agree on that.”
Because FRBs can vary in brightness, duration and other properties, it is unlikely that any single observation can explain them all. In the summer of 2021 the Canadian Hydrogen Intensity Mapping Experiment (CHIME), a dedicated FRB-hunting telescope in British Columbia, released a catalog of 536 FRBs that it had detected during the first year of its operation, quadrupling the number known. The bursts were already known to come in two distinct flavors—those that repeatedly flash their signals and those that are one-off events. CHIME's data showed that nonrepeaters were far more common than repeaters and that each had different characteristics.
On average, the bursts from repeaters lasted longer than their one-off counterparts and emitted their light in a narrower range of frequencies. Whether the disparity represents a different mechanism for these flashes or something else about their progenitors' ages or environments remains to be seen. The situation resembles an earlier mystery surrounding another class of cosmic explosions: gamma-ray bursts, which were shown in the 1990s to arise from three separate types of events. Scientists hope to discover whether FRBs also fall into distinct populations with their own origin stories.
CHIME's catalog includes large numbers of FRBs from a variety of galaxies, muddying the link to magnetars, which emerge almost exclusively in galaxies that are churning out massive, short-lived stars. CHIME's FRB haul, however, includes many sources from quieter galaxies that are barely forming any new stars at all. “Magnetars can explain some fraction of FRBs. Nobody would dispute that,” says Shami Chatterjee, an astronomer at Cornell University. “But is that all of them? Almost certainly not.”
A paper published in Nature in February 2022 adds support to this assertion. Using an array of radio telescopes called the European Very Long Baseline Interferometry (VLBI) Network, a team determined the position of a repeating burst designated FRB 20200120E with extreme precision. The object had originally been localized to the nearby spiral galaxy M81, but VLBI zoomed in farther and revealed that it lives within an ancient hive of densely packed stars known as a globular cluster. Such collections mainly host stars around 10 billion years old—yet magnetars are thought to endure for only 10,000 years or so before lapsing into more sedate neutron stars. “This is a game changer,” says Mohammadtaher Safarzadeh, a theoretical astrophysicist at Harvard University. “Whatever is causing the FRB signal likely has the same age as the globular cluster and is definitely not a magnetar.”
Magnetars might occasionally arise from two neutron stars crashing into each other—a production mechanism that has never been confirmed—which could potentially explain one in a globular cluster, says theoretical astrophysicist Bing Zhang of the University of Nevada, Las Vegas. But nobody knows exactly how often such events occur or how long the resulting magnetars would remain active.
Further complicating the magnetar picture is another curiosity: FRB 20180916B, also known as R3 because it was the third repeating FRB ever discovered. Originally pinpointed to a region toward the star-forming center of a spiral galaxy around half a billion light-years away, R3 was subsequently shown to be in the galaxy's outskirts, suggesting that it is either an older object or one somehow kicked away from its birthplace. Even stranger, this burst produces explosions only during a four- to five-day window of activity that occurs every 16.35 days, making it a so-called periodic repeater.
Researchers have been scratching their heads over its peculiar regularity. A magnetar that spins around on its axis like a top, sometimes pointing its blasts toward Earth and other times facing away, is one possible explanation. Another is a bursting object orbiting a second structure, such as a black hole surrounded by a disk of material, that cyclically obscures the explosive events. Scientists have even suggested it comes from a pair of orbiting neutron stars whose magnetospheres periodically interact, creating a cavity where eruptions can take place. “What makes the field so fun right now is that there are so many exciting possibilities,” Chatterjee says.
FRB astronomers are still pursuing major questions. Are nonrepeaters really one-time events, or could they burst again if we watch for long enough? The magnetar in our galaxy appears to be fairly quiet. But was it significantly more active in its younger years? Could other esoteric scenarios, such as asteroids hitting a black hole, somehow produce FRB-like signals? Scientists are publishing new observations and theories all the time.
The CHIME collaboration is building a set of smaller telescopes that will help triangulate the exact positions of many FRBs. In a few years researchers expect to know the precise locations of hundreds or even 1,000 events. In addition to elucidating FRBs, these data will allow scientists to perform important measurements of the universe.
Astronomers first knew FRBs were coming from outside the Milky Way because their light was dispersed, meaning the higher frequencies arrived a few milliseconds before the lower ones. This pattern offers information about the matter the radio waves traveled through as they made their way through space. Astronomers believe there is much more regular matter in the universe than what we see in stars and galaxies, and they suspect that the missing matter lies in the intergalactic medium. In 2020 a team studied a handful of FRBs to estimate how much material their light passed through and showed it was almost exactly equivalent to the amount of matter expected there.
The ultimate goal is to use FRBs to map the matter throughout the universe. And light from some FRBs is highly polarized—its waves have been rotated by magnetic fields during its flight—potentially revealing information about magnetic conditions in other galaxies or the spaces between them. In the meantime, the mystery of FRBs' origins remains. “I fully anticipate, within the next decade, we'll get one or two more surprises, like the galactic magnetar that we didn't even know we should be looking for, which will push our understanding forward in a massive way,” Petroff says.
If some nonrepeating FRBs arise from cataclysmic events such as neutron stars crashing together, as many astronomers suspect, they would also create gravitational waves. Were a radio telescope to see a blast at the same time as the Laser Interferometer Gravitational-wave Observatory (LIGO) or its counterparts around the world, it would sway some toward that possibility. And if such a collision produced a magnetar, could the initial cataclysmic one-off FRB give rise to a distinct, repeating FRB source? Time will tell.
Given recent history, more FRB excitement is likely in the coming years, Lorimer says: “Just when you think things are settling down, you have a year with all these remarkable discoveries.”