“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