A new telescope has revealed an important clue in the hunt for the astrophysical sources of powerful, enigmatic radio bursts. Canada’s Hydrogen Intensity Mapping Experiment (CHIME), inaugurated last fall at a remote site in British Columbia, has spotted a new burst at a lower frequency than any previous detections. The new discovery should provide insight into the elusive origins of the strange bright signals, and augurs a dawning era in which they will be found and studied by the thousands.
Fast radio bursts (FRBs) are blasts of energy that span just a fraction of a second but can shine brighter than half a billion suns at radio wavelengths. First identified in 2007, FRBs appear randomly on the sky, their light so fleeting scientists have struggled to trace them back to any obvious source. Of the 35 previously known bursts, only one has been found to repeat—offering astronomers a chance at more detailed studies, and confirming some FRBs must come from sources that somehow survive the extreme events required to produce such intense energies. This repeating FRB was eventually traced to a region of intense star formation inside a distant dwarf galaxy. All the others, however, remain tantalizing one-offs, despite astronomers’ efforts to watch for more repetitions.
CHIME is poised to change all that. Its quartet of 100-meter-long cylindrical reflectors were built to create three-dimensional maps of hydrogen gas across much of the observable universe, but also serve as potent FRB detectors. Whereas most telescopes study a small slice of the sky, CHIME scans the entire northern celestial hemisphere every day, studying each section in 15-minute intervals. “This is the real key of CHIME,” says CHIME collaboration member Patrick Boyle, of McGill University. “Nobody else has that field of view.”
Such an expansive eye on the sky could soon provide more FRB data than astronomers can easily handle. According to Shriharsh Tendulkar, a CHIME researcher also at McGill, the telescope should detect anywhere from one to 10 FRBs each day, revealing as many as 3,500 FRBs in a given year.
‘Very Much an FRB'
On July 25, CHIME detected the burst now labeled FRB 180725A, and announced its discovery on August 1. CHIME’s inaugural FRB is of particular interest because it was detected at lower frequencies than any of its predecessors. Although most FRBs have been detected at frequencies ranging from 800 megahertz (MHz) to 1 gigahertz (GHz), CHIME’s signal dipped as low as 580 MHz—more than 100 MHz lower than the previous record-holding FRB. And because CHIME’s survey of the radio sky reaches as low as 400 MHz, it may soon yield additional FRBs at even lower frequencies.
CHIME’s emphasis on relatively low-frequency radio waves made some scientists doubt the instrument would spot FRBs at all, says Cornell University astronomer Shami Chatterjee, who is not part of the CHIME team. Perhaps, some thought, FRBs simply would not emit detectable signals at such low frequencies. “There was a lot of worry before CHIME turned on. What if they turned it on and they don’t actually see any FRBs?” Chatterjee says. “I think that has been laid to rest.”
CHIME has been studying the night sky since last November, gradually increasing its view and effectiveness. It identified the new signal only a few days into its latest development cycle, startling the CHIME team. “We were all initially surprised and skeptical,” says McGill’s Emmanuel Fonseca, a CHIME collaborator. During its observations, CHIME generates 13,000 gigabits per second—a deluge of raw data the team is still working to tame. Mistakes, they knew, could happen. But a careful review of the data and the telescope’s operations rapidly convinced the team the signal was genuine. “This FRB was very much an FRB, and we were all very much excited,” Fonseca says.
Because it detects FRBs at lower frequencies than other instruments do, CHIME should help researchers better understand how an FRB’s radio waves interact with interstellar material and magnetic fields as they travel through space. “That has importance for how energetic they are and how FRBs are made,” says Emily Petroff, a researcher at ASTRON Netherlands Institute for Radio Astronomy who is unaffiliated with CHIME. “The fact that CHIME is able to probe this lower-frequency domain is really useful. It’s showing us this new limit on how low-frequency FRBs can be.”
Remarkably, when the telescope spotted FRB 180725A, it was far from full capacity, using less than half of its available sensors, Boyle says. When fully operational, it should transform the field of FRB studies by detecting multiple bursts every day. The previously observed FRBs should find themselves rapidly outstripped by CHIME’s discoveries, giving scientists more opportunities to study and understand the puzzling flashes. “It took a little over 10 years to detect  FRBs, and CHIME is poised to basically blow that number out of the water on a very short timescale,” Fonseca says.
Researchers are especially eager for CHIME to uncover other repeating FRBs, to compare them with the lone repeater already known. It may be that all FRBs repeat due to their sharing a common, universal astrophysical origin; or, perhaps, only some do, hinting that the universe has many ways of producing FRBs—some self-destructive, some not. “Because [CHIME] covers such a huge field of view every day, the chances of finding repeated bursts are much higher,” says Maura McLaughlin. A researcher at West Virginia University, she has been using the Green Bank Telescope to hunt for low-frequency FRBs in small, pinholelike patches of the sky. Despite her role with a separate team, she is excited about the new detection. “Within the first few months of observation, we will know if most FRBs repeat or not.”
A Puzzling Source
Not knowing the source of FRBs has not stopped scientists from speculating. According to McLaughlin, the most favored explanation for any given FRB is a pulsar, a dense stellar core that spins like a lighthouse beacon, only far faster. “[The FRB signals] look a lot like radio pulsar pulses, they’re just much brighter than the pulses from pulsars in our galaxy,” she says.
Pulsars in our own galaxy are already bright in the frequencies at which most FRBs have been observed, but these familiar pulsars shine even brighter at the lower frequencies CHIME is only now beginning to probe. From its perch high in the Northern Hemisphere CHIME can see approximately two thirds of the previously identified FRBs, allowing it to check for repeaters at lower energy levels than ever before. “With FRBs, it would be interesting to know if they are also brighter at lower frequencies,” Petroff says. “That would be a more direct link to the pulsar population in our own galaxy.” It is also possible, she adds, CHIME will not turn up many low-frequency FRBs at all—which would not rule out pulsars as a potential source, but could push them out of favor. Other possible explanations include cataclysmic events such as colliding neutron stars, pulsars collapsing to form black holes or particularly violent flares from “magnetars”—pulsars with extreme magnetic fields.
Sooner or later CHIME and other upcoming instruments should help solve the mystery of fast radio bursts. “With their powers combined, we’re going to find thousands of FRBs in the next few years,” Petroff says. “CHIME is signaling the beginning of this era, and I think that’s really exciting.”