Fast radio bursts, or FRBs, are one of the greatest mysteries of our universe. Coming from deep space, these outbursts can flash and fade in a matter of milliseconds, yet in each instance can release as much energy as the sun does in a year. They pop up all across the sky multiple times a day, but most appear to be one-off events and are thus hard to catch. First discovered in 2007, FRBs have challenged and tantalized scientists seeking to uncover their obscure origins and to use them as unique tools for probing the depths of intergalactic space.
Now, using the world’s largest single-dish radio telescope, an international team has reported the largest set of FRB events ever detected in history. According to their paper published in Nature today, between August and October 2019 the Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST) in southwestern China recorded a total of 1,652 such brief and bright outbursts from a single repeating FRB source in a dwarf galaxy three billion light years away. Besides dramatically boosting the total number of known FRB events, the observations also revealed a very wide range of brightnesses among the recorded events, offering new clues about the astrophysical nature of their mysterious source.
“The study is very thorough, with a level of details and sensitivity we’ve never had before,” says astrophysicist Emily Petroff from the University of Amsterdam in the Netherlands and McGill University in Canada, who is not involved in the research. “Such in-depth analyses of individual sources will be a top priority in FRB research in the near future.”
A Bevy of Bursts
The first FRBs struck astrophysicists like thunderbolts out of a clear blue sky; no theory had predicted their existence. Early on, researchers had little clue what the bursts could be, and scrambled to come up with ideas. Explanations for FRBs have ranged from enormous magnetic eruptions upon spinning neutron stars to the emissions from star-hopping alien spaceships. For a time—before FAST and other FRB-hunting telescopes began operations, anyway—the running joke among theorists was that FRB theories outnumbered the known FRB events themselves.
It was not until 2016 that observers detected the first repeating source, named FRB 121102. Statistics drawn from the ever-expanding catalog of detections have now revealed that about 20 percent of FRBs happen more than once, and these repeating sources allow astronomers to make more detailed follow-up observations. FRB 121102 is the best studied such source so far. Prior to FAST’s mother lode of new events, scientists using other radio telescopes had reported nearly 350 FRBs from this source, which is nestled in a galaxy where lots of young stars are taking shape. “With a repeating source, other telescopes usually get somewhere between two and a hundred pulses. FAST did more than one thousand, which is amazing,” Petroff says.
Thanks to the unprecedented sensitivity of FAST, it can catch less energetic pulses that other telescopes cannot, says Di Li, the paper’s lead author and FAST’s chief scientist. When the team performed test observations during the telescope’s commissioning phase, they noticed that FRB 121102 was in a frenzy of activity, frequently emitting bright pulses. So, they decided to spare about an hour every day to monitor it. The bursts turned out to be much more intensive than expected. During some episodes, there was about one every 30 seconds.
The bursts fell into two types: ones with high brightness and others with low brightness. This may point to two distinct physical mechanisms that are responsible for the bursts, says study co-author Duncan Lorimer, of West Virginia University, who co-discovered the first FRB in 2007.
It is not yet clear, however, what those mechanisms are. Even so, because the ensemble of pulses exhibited such high energies and did not show any short-term periodicity (which would suggest a source that spins or orbits at a set pace), Li believes that he and his collaborators have severely constrained the possibility that FRB 121102 comes from an isolated compact object such as a rotating neutron star or a black hole.
Others hesitate to draw the same conclusion. For instance, FRB 121102’s source could still be a magnetar, a special type of neutron star with an extremely strong surface magnetic field, says theoretical physicist Zigao Dai from the University of Science and Technology of China in Hefei. Magnetars can experience “starquakes” when their outer layers adjust under stress caused by sudden shifts in stellar magnetic fields. Just like an earthquake on Earth can be triggered by different mechanisms, such as the motions of tectonic plates or the impact of an asteroid, “it remains possible for a magnetar, for instance, to go through starquakes and to frequently get hit by asteroids around it as well—a probable scenario in the galaxy [FRB 121102] lives in,” Dai explains.
FRBs on the FAST track
“FAST is really great at studies like this one—in-depth analyses of repeating sources,” Lorimer says. While it is not especially adept at finding FRBs, its enormous sensitivity allows it to detect things that other telescopes miss. This is why for FRB studies FAST works best in tandem with other radio telescopes, such as the Canadian Hydrogen Intensity Mapping Experiment (CHIME), which is a powerhouse for spotting FRBs anywhere in the overhead sky thanks to its vast field of view.
Earlier this year, FAST announced its second open call for proposals, with 15 to 20 percent of the telescope’s total observing time made available to the international community. FAST was completed in 2016, superseding the iconic Arecibo Telescope in Puerto Rico as the world’s largest single-dish radio telescope.
Petroff, who is a member of the CHIME/FRB collaboration, says her team has now applied for and been rewarded observing time on FAST. According to Li, observations for approved international programs have already begun. As international travel is still restricted because of COVID-19, foreign scientists are for now limited to remote operations, and are required to submit a proof of identity, typically a copy of their passport information page, for access.
“We’ve been working with individual scientists to reduce their concerns and explore alternative ways of submitting personal information,” Li notes. “The FAST staff warmly welcome them to come and visit once international travel normalizes, hopefully soon.”
FAST will keep monitoring FRB 121102 while looking into other repeating sources, Li says. In fact, he teases, his team has been working on another source, yet to be publicly revealed, that behaves “more radically” than FRB 121102. Studying run-of-the-mill as well as “radical” FRB systems, Dai says, is crucial to understanding what is and is not possible for FRBs—and thus what their true nature must be. Making further breakthroughs, he and other experts say, probably requires the coordinated efforts of multiple telescopes around the globe observing in many different types of celestial light—as well as in neutrinos and gravitational waves, too.
“I’d say FRB astronomy is still in an adolescent phase,” says Lorimer. “We know quite a lot about FRBs, but there are still a number of ‘growing pains’ with many of the theories.” The next step is to continue to pinpoint home galaxies for as many sources as possible, carrying out in-depth analyses of individual systems as Li and his team have done with FAST. With considerable effort and, perhaps, a bit of luck in finding more frenzied repeaters and radical one-off FRBs, scientists may soon solve the deep cosmic mystery of FRBs, and open a new window on the high-energy, short-lived astrophysical phenomena that fill the universe.