Fast radio bursts, or FRBs, are intense, millisecond-long bursts of radio waves of unknown origin. The first FRB was discovered in 2007, and since then hundreds of these fast, cosmic flashes have been spotted coming from various distant points across the universe.
Many FRBs emit super-bright radio waves that last at most a few milliseconds before completely disappearing, and about 10% of them are known to be repetitive and patterned.
One source used to discover them is a radio telescope called the Canadian Hydrogen Intensity Mapping Experiment, or CHIME, at the Dominion Radio Astrophysical Observatory in British Columbia, Canada.
This telescope, which has been in operation since 2018, constantly monitors the sky and is sensitive to radio waves emitted by far away hydrogen in the universe, in addition to rapid radio bursts.
Astronomers using CHIME discovered something on December 21, 2019 that immediately caught their attention: a fast radio burst that was “peculiar in many ways,” according to Daniele Michilli, a postdoctoral researcher at the Kavli Institute for Astrophysics and Space Research of the Massachusetts Institute of Technology.
Dubbed FRB 20191221A, the signal lasted up to three seconds — about 1,000 times longer than typical rapid radio bursts.
Michilli was monitoring the data coming in from CHIME as the outbreak occurred. The signal is the longest-lasting rapid radio burst to date.
“It was unusual,” Michilli said. “Not only was it very long, lasting about three seconds, but there were periodic spikes that were remarkably precise, sending out every split second — boom, boom, boom — like a heartbeat. This is the first time that the signal itself is periodic.”
While FRB 20191221A has yet to repeat itself, “the signal is formed by a series of consecutive spikes that we believe are separated by ~0.2 seconds,” he said in an email.
An unknown source
The research team doesn’t know the exact galaxy where the burst originated, and even the one billion light-year distance estimate is “highly uncertain,” Michilli said. While CHIME is primed to look for bursts of radio waves, it’s not that good at locating their points of origin.
However, CHIME is being upgraded by a project where additional telescopes currently under construction will observe together and be able to triangulate radio bursts to specific galaxies, he said.
But the signal contains clues as to where it came from and what might have caused it.
“CHIME has now discovered many FRBs with different properties,” Michilli said. “We’ve seen some living in very turbulent clouds, while others look like they’re in a clean environment. Based on the properties of this new signal, we can say that around this source there must be a plasma cloud that is extremely turbulent.”
When the researchers analyzed FRB 20191221A, the signal resembled emissions released by two different types of neutron stars, or the dense remnants left after the death of a giant star, called radio pulsars and magnetars.
Magnetars are neutron stars with incredibly strong magnetic fields, while radiopulsars emit radio waves that appear to pulse as the neutron star spins. Both star objects produce a signal similar to a lighthouse flash.
The fast radio burst appears to be more than a million times brighter than these emissions. “We think this new signal could be a magnetar or a pulsar on steroids,” Michilli said.
The research team will continue to use CHIME to monitor the sky for additional signals from this burst, as well as others with a similar, periodic signal. The frequency of radio waves and how they change could help astronomers learn more about the expansion rate of the universe.
“This discovery raises the question of what could be causing this extreme signal that we have never seen before and how we can use this signal to study the Universe,” Michilli said. “Future telescopes promise to discover thousands of FRBs every month, and at this point we could find many more of these periodic signals.”