SEATTLE, WASHINGTON—A new Canadian radio telescope, not yet fully operational, has already detected more than a dozen of the mysteriously brief blasts from deep space known as fast radio bursts (FRBs). One is only the second known to flash repeatedly, researchers reported here today at the annual meeting of the American Astronomical Society. The early results from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) suggest the scope is well on its way to adding hundreds or even thousands of FRBs to the 60 or so already known—hopefully revealing the source of these powerful millisecondslong pulses in the process.
“This really points to the fact that CHIME is set to revolutionize the field of FRBs,” says Sarah Burke-Spolaor of West Virginia University in Morgantown, who was not involved in the research.
FRBs are one of the hottest topics in astronomy. Researchers not only want to figure out what they are, they also want to use them to gather information about the matter that resides in the vast reaches between galaxies. As they journey through deep space, FRB pulses get spread out by all the electrons they meet, revealing information about the density of the intergalactic medium. That would be valuable input for models of the large-scale structure of the cosmos. “FRBs could be a good way to understand the evolution of our universe,” says Vishal Gajjar of the University of California, Berkeley, also not a member of the CHIME team.
FRBs were first detected in 2007 by telescopes in Australia. For years, skeptical astronomers dismissed them as local effects or instrumental glitches. Because FRBs are rare, only wide-field telescopes have a chance of catching one. But these survey scopes tend not to be sensitive enough to learn much about them. And because FRBs occur in the blink of an eye, it’s too late to bring another, more sensitive, telescope to bear on it.
Astronomers began to take FRBs seriously when, earlier this decade, teams figured out that the pulses came from distant galaxies. That discovery was based on the structure of the pulses themselves: Among the range of frequencies that make them up, longer wavelength photons lag behind the shorter ones, thanks to the drag of intergalactic matter. The amount of lag in an arriving pulse is too great for the FRB to be from a source within the Milky Way. Previously, some scientists thought explosive events in our galaxy such as supernovae or neutron star mergers might be responsible for the bursts.
But in 2012, an FRB was found by the Arecibo Observatory in Puerto Rico that was later shown to repeat. This ruled out one-off sources like mergers or supernovae that would be consumed in the process—for that FRB at least. Further observations with the Green Bank Telescope in West Virginia told researchers that the burst, known as FRB 121102, came from a highly magnetic environment. In 2017 researchers used the Very Large Array in Socorro, New Mexico, and the European VLBI Network—a continent-wide array—to pin down its location to a small star-forming galaxy 3 billion light-years away.
But what spawns FRBs remains a mystery. There are almost as many theories as there are FRB detections. An online list now has 47 entries, including neutron star-white dwarf mergers, lightning on pulsars, and alien light-sails. But with only 60 FRBs, astronomers have little to go on. Finding more FRBs—and more repeaters—will let researchers statistically analyze them, and perhaps even determine which types of galaxies spawn them.
CHIME, originally designed to map clouds of interstellar hydrogen to understand the mysterious dark energy that is accelerating the expansion of the universe, aims to help. The telescope, near Penticton in British Columbia in Canada, is comprised of four, fixed 100-meter-long parabolic troughs that look straight up and scan the whole visible sky more than 24 hours.
Construction was finished in 2017. In July and August 2018, while parts of the system were still being tested, CHIME bagged 13 new FRBs over 3 weeks, including the second repeater. “It was a happy surprise, with an element of relief, too,” says Ingrid Stairs of the University of British Columbia in Vancouver, one of the leaders of the CHIME FRB team. Previously, no FRBs had been found at frequencies below 700 megahertz (MHz), and scientists were worried that not many FRBs would be visible in CHIME’s 400- to 800-MHz range. Shriharsh Tendulkar of McGill University in Montreal, Canada, lead author of one of two CHIME papers published today in Nature, says they want to detect across as broad a range of frequencies as possible, both to catch more FRBs and to better understand what is producing them.
Burke-Spolaor says the second repeater is exciting because it confirms their existence and heralds more discoveries. Researchers can’t yet tell whether repeaters are a distinct type of FRB or a stage in their long evolution: Single FRBs, for example, could actually be repeaters that have slowed with age and burst too rarely for us to see repeats. The two known repeaters show noticeable similarities, with more structure in their pulses—a series of subbursts—than all but one of the single FRBs. “The striations in the pulses are so rich in information,” Burke-Spolaor says. “Finding more repeaters is very important because they are easier to localize [to a source galaxy].” CHIME’s results support the idea that FRBs come from dense star-forming regions and perhaps from within old supernova remnants.
Researchers are already looking forward to the haul that CHIME should return when it comes online later this year. Gajjar says: “We should get busy.”