First we detected them. Then we proved they came from space. Next we located the cosmic birthplace of one of them. Now we have located that of another—bringing humanity closer than ever to solving yet another mystery of the universe. Fast radio bursts (FRBs) have, since their discovery a decade ago, confused and befuddled astronomers. These strange blasts of radio waves appear in the sky from all directions, and their origins remain mostly unknown. But in this scientific quest, each new FRB astronomers detect and study is one more puzzle-piece falling into place; eventually, experts say, the full picture will be revealed. “The analogy is climbing a mountain,” says James Cordes of Cornell University. “We’ve climbed quite a ways. But the peak is still way above us.”
In the June 27 edition of the journal Science, astronomers led by Keith Bannister of the Commonwealth Scientific and Industrial Research Organization (CSIRO) in Australia took another crucial step toward that lofty peak. The team announced it had successfully pinpointed the home of an FRB some 3.6 billion light-years from Earth, the second such discovery made. Following earlier observations, in 2017 the first home of an FRB—called FRB 121102—was tracked down to a star-forming region in a dwarf galaxy slightly closer to us, only three billion light-years away. There are two major curious differences this time around, however. Unlike its predecessor, this latest FRB, known as FRB 180924, is nonrepeating, making localization particularly tricky. Also, and perhaps more importantly, the newer FRB comes from a very different type of galaxy, which could have major implications for how FRBs are created.
“[FRB 180924’s home] is very different from the only known other host galaxy,” Bannister says. “It’s about 1,000 times bigger than that galaxy, or a bit smaller than the Milky Way, and that’s really interesting. Because the first [FRB-hosting] galaxy that was discovered was really vigorously forming stars. So this one is a big galaxy, and it’s a bit surprising that it can form an FRB, because we thought that FRBs came from special places. What we’re seeing here is that quite benign, normal-looking galaxies can produce FRBs, too.”
The first FRB was discovered in 2007 by then physics undergraduate student David Narkevic, astronomer Duncan Lorimer and their colleagues, using the Parkes radio telescope in Australia. Studying data from 2001, they spotted a bright burst of radio waves lasting just five milliseconds, which seemed to originate from a region of the sky near the Small Magellanic Cloud, a satellite dwarf galaxy of the Milky Way. Before long, other teams of astronomers began to find FRBs of their own in Parkes data. A subsequent batch revealed in 2013 hinted that FRBs were truly cosmic phenomena, with some seeming to occur as far as 10 billion light-years from Earth. As the detections piled up, it became clear that whatever FRBs were, they certainly were not rare but rather occurred regularly across the entire sky. Astronomers’ arduous journey from base camp had begun.
As quickly as FRBs accumulated, so, too, did theories about what caused them. “For a long time we joked that there were more models for FRBs than detected FRBs,” says Shami Chatterjee of Cornell. Human-caused radio interference (such as errant signals from overhead satellites or even nearby microwave ovens) was one of the earliest suggestions—particularly because at the time all the known FRBs had been found only via the Parkes telescope. That theory was quashed when scientists began finding FRBs with other telescopes, such as the Arecibo Observatory’s giant radio telescope in Puerto Rico, which discovered FRB 121102 With terrestrial sources eliminated, astronomers focused on more otherworldly explanations, speculating that FRBs might be burps from evaporating black holes or even intergalactic transmissions from alien civilizations.
FRB 121102 complicated matters, however, when in 2016 it was shown to repeat. That repetition carried with it an important implication: some FRBs—perhaps all—could not be caused by one-off cataclysmic events such as colliding stars or expiring black holes. Instead they would need to emerge from nondestructive processes, such as rare outbursts from magnetars, rapidly spinning and usually newborn neutron stars that boast immensely powerful magnetic fields. Now FRB 180924 has complicated matters further, because its source appears to be a relatively sedate, Milky Way–like galaxy filled with old stars, as opposed to the more active galaxy for the prior localized FRB, where magnetars would be more likely to be found. “For that reason, you wouldn’t really favor the magnetar model,” Bannister says. “This observation doesn’t rule it out, but it certainly calls into question whether that model is useful for this case.” Instead, Bannister notes, some even more exotic variety of neutron star outburst or one-off collision model may be required.
Pinpointing the location of this FRB was no mean feat. For a repeating FRB, astronomers know exactly where to look with telescopes to monitor and then follow up on each individual flash. But localizing nonrepeating FRBs requires that researchers observe large chunks of the night sky to detect one and then quickly investigate with other telescopes. In this case, Bannister and his team first used the 36 antennas of the Australian Square Kilometer Array Pathfinder (ASKAP) radio telescope to spot the FRB, next turning to the Very Large Telescope in Chile and the Keck telescopes on Mauna Kea in Hawaii to perform additional observations that revealed its host galaxy. (It is worth noting that FRB 180924 may well repeat, albeit at timescales and intensities that presently preclude further detections. Regardless, the challenge of locating it is the same.)
With detailed studies of FRBs requiring so many telescopes, one might wonder why such intensive efforts would be justified. The answer is that these mysterious outbursts could be used as unique probes of matter between galaxies. As the radio waves from FRBs travel to Earth, they pass through wisps of ionized gas that exist throughout intergalactic space and become distorted, based on just how much stuff they encounter along the way. “The radio waves from the burst are affected by this,” says Ryan Shannon of the Swinburne University of Technology in Australia, a co-author of the new paper. “We’re basically able to count the entire density of electrons along the line of sight. And that allows us to measure the total density of the material.”
By localizing more and more FRBs, astronomers hope to learn not only about the phenomenon itself but also about the universe at large, building up these elusive events as unprecedentedly powerful probes of cosmic structure and evolution. “A sample of two is not terribly useful,” says Emily Petroff of the University of Amsterdam. Even so, she says, “it’s actually quite interesting that the first two host galaxies identified could not be more different from each other.”
Multiple projects are now focused on finding more and more FRBs, chief among them the Canadian Hydrogen Intensity Mapping Experiment (CHIME) radio telescope. But many lack the precision of ASKAP, meaning there is still a substantial challenge ahead to track down the fundamental sources and causes of FRBs. “I would say this result is complementary to CHIME,” says Cherry Ng of the University of Toronto, a member of the FRB-hunting CHIME team. “CHIME's localization capability is limited. On its own, [it’s] not able to pinpoint the host galaxy. ASKAP might see fewer FRBs, but they are able to provide precise localization.”
For now, the FRB mystery persists, but scientists say they are creeping ever closer to a breakthrough. Astronomers at CHIME and other projects are forecasting the discovery of hundreds of additional FRBs in 2019 alone. The fact that this prediction is perhaps rather conservative is a testament to how quickly the field has exploded in just a decade. The summit may be within reach sooner rather than later. “We are not talking decades,” Chatterjee says. “In a few years we will have a whole bunch of these localizations. Then it’s up to the theorists to come up with mechanisms.”