Repeating fast radio bursts remain a mystery for astronomers, but these new discoveries could lead to key answers about them, and provide insights about other mysteries of the cosmos as well.
Fast radio bursts, or FRBs, are brief, powerful pulses of radio waves detected from space. Some can last up to three seconds long, while others appear and disappear in a fraction of a millisecond. However, their origin is a mystery. Given the amount of energy they carry, researchers speculate that they are produced by some of the highest energy events in the universe — supernovae, gamma-ray bursts, or collisions between neutron stars, pulsars, or black holes. The only thing that is known for sure is that most FRBs originate from outside our galaxy.
This artist's impression shows a fast radio burst travelling between its source in a distant galaxy (top left) towards Earth in the Milky Way (bottom right), passing through the halo of a massive galaxy along the way. Credit: ESO/M. Kornmesser
It's been over 15 years since the first FRB was detected from space. In that time, hundreds more have been found, but still, astronomers are no closer to figuring out exactly what causes them.
Even more puzzling are the few FRBs found that periodically repeat. Until now, of the hundreds of FRBs detected, only 25 belonged to a particular class known as repeating FRBs.
Finding what was missed
In new research, a Canadian-led team of astronomers turned up another 25 repeating FRBs, doubling the number already discovered.
The researchers found them by performing the very first delve through all of the data gathered between September 2019 and May 2021 by the Canadian Hydrogen Intensity Mapping Experiment. CHIME is a unique, highly-sensitive radio telescope at the Dominion Radio Astrophysical Observatory near Penticton, British Columbia, located on the traditional, ancestral, and unceded territory of the Syilx/Okanagan people.
The four 'cylinders' of the CHIME radio telescope sit fixed in place, staring up at the sky from the floor of southern B.C.'s Okanagan Valley. Credit: CHIME Collaboration
"Many apparently one-off FRBs have simply not yet been observed long enough for a second burst from the source to be detected," said Dr. Ziggy Pleunis, a postdoc researcher at the University of Toronto's Dunlap Institute for Astronomy and Astrophysics who is one of the nearly 60 scientists involved in this new study.
"We need a longer observation time because some repeaters could repeat every 10 years. We just don't know. They don't play by our time scales," added co-author Adam Dong, a Ph.D. student in the University of British Columbia's department of physics and astronomy.
Of the 25 newly discovered repeating FRBs, most were spotted two or three times during CHIMEs observations. During that same time, one of them — FRB 20201124A, first spotted in 2020 and found to originate from a nearby galaxy — was seen to repeat a total of 12 times!
This map of the sky, taken from the new research study, shows the locations of all repeating fast radio bursts detected so far. Credit: CHIME/FRB Collaboration/The Astrophysical Journal
Picking out these signals required the team to develop new statistical tools to sift through CHIME's data.
"We can now accurately calculate the probability that two or more bursts coming from similar locations are not just a coincidence," Pleunis explained. "These new tools were essential for this study, and will also be very useful for similar research going forward."
One of the challenges of studying FRBs is that there's no predicting when one will appear. In most cases, astronomers can only point their radio telescopes at the sky and hope that they pick up one or more of these signals during their observation time. Some researchers have predicted that thousands could be going off every day over the entire sky. However, we only detect a small number due to the limited amount of the sky current radio telescopes can scan at any time.
Finding repeating FRBs is even more tricky. This is because radio telescopes must be pointed at the same portion of the sky during each repeated signal. So, without knowing the timing of the repeats, it becomes even more dependent on luck.
CHIME sweeps for FRBs
In operation since 2017, CHIME observes the entire sky above it all at once, ready to intercept any signals from space that appear in its field of view. Also, while optical telescopes typically need to wait until dark to observe, radio astronomy can be conducted day or night. Thus, as Earth rotates, CHIME can sweep through the entire northern half of the celestial sphere each day.
In its first year alone, CHIME picked up over 500 FRBs. According to the CHIME Collaboration, by mid-2020, the telescope had detected well over 1,000.
Watch below: A time-lapse of one full day of CHIME observations
CHIME is an excellent tool for detecting FRBs, but it does have its limitations. Since it is tied to the rotation of Earth, the telescope's field of view sweeps around space, a bit like the cone of light from a lighthouse. Thus, while it can cover the entire northern celestial sphere in a day, how many FRBs it detects and how many repeating FRBs it finds depends on exactly what part of space it is observing at any one time. If the timing of an FRB — repeating or not — is off by even the smallest amount, such that the source is below the horizon from CHIME when the signal arrives here, the telescope will still miss it.
However, if there were more telescopes like CHIME, astronomers could cover much more space at once, thus catching far more FRBs and discovering more repeating ones.
Why is this important?
The researchers believe their new techniques will help find even more repeating FRBs. Other telescopes can then observe those discoveries at just the right time to pick up the repeat signals.
"FRBs that repeat are great targets for other telescopes, including those that can measure their positions very accurately, and let us know which galaxies they come from," said co-author Dr. Ingrid Stairs, a professor in the University of British Columbia's department of physics and astronomy, according to UBC News. "In the long run, we hope to learn a lot about their origins."
"FRBs are likely produced by the leftovers from explosive stellar deaths." Pleunis said, referring to neutron stars, pulsars, and black holes, or phenomena such as gamma-ray bursts. "By studying repeating FRB sources in detail, we can study the environments that these explosions occur in and understand better the end stages of a star's life. We can also learn more about the material that's being expelled before and during the star's demise, which is then returned to the galaxies that the FRBs live in."
Also, detecting more repeating FRBs can help astronomers discover the answers to other questions about the universe.
"One exciting avenue of research is utilizing them to measure the amount of matter between galaxies, or the intergalactic medium," Adam Dong explained in the UBC press release.
Additionally, besides the 25 confirmed repeating FRBs found in this study, the researchers identified another 14 possible candidates. While there were significant enough differences between repeated bursts for these candidates — in position, dispersion, timing, etc. — if they can be confirmed as actual repeaters, it could reveal even more about these mysterious phenomena.