Mysterious fast radio bursts (FRBs) originating from deep space may be repeat offenders rather than one-off events, new research using Canadian facilities suggest.
FRBs are transient pulses of radio wavelengths lasting anywhere from a fraction of a millisecond to three seconds long, but it has been difficult to track down their source or understand more about their origins. One great form of assistance, however, is a Canadian facility known as CHIME (Canadian Hydrogen Intensity Mapping Experiment).
CHIME is located at the Dominion Radio Astrophysical Observatory near Pencticton, B.C. and was originally used for mapping the abundance and location of hydrogen in the sky. The telescope arrow has both a large, instantaneous field of view (of roughly 200 square degrees) and a large number of frequencies it can monitor (between 400 and 800 MHz).
The range and sensitivity of the telescope are both growing as “outrigger” observatories are added, which means that CHIME is also useful in a fast-growing field: seeking out and understanding FRBs.
“FRBs are exciting for the mystery that we’d like to solve … but they also will have very nice applications as well because we can use them to study all of the material in between their location and ours,” participating author Ziggy Pleunis told SpaceQ. Pluenis started working on the research as a PhD student at McGill University. (He is now a Dunlap Postdoctoral Fellow at the Dunlap Institute for Astronomy and Astrophysics.)
“They are coming from galaxies, sort of far away, and they interact with all of the material in between,” he added. “We can use them to measure the density of other galaxies and magnetic fields.”
The new study, published in The Astrophysical Journal and led by McGill’s Bridget C. Andersen, narrows in on 25 repeating FRBs detected between September 2019 and May 2021, along with 14 “candidate” repeating FRBs that the team hopes that other studies will examine with more wavelengths and observatories. CHIME is attributed as key to the study’s success due to its “large field of view, high sensitivity, and daily surveying of a large area of sky”, the study authors write.
The team used a dataset of CHIME detections and obtained a list of potential candidates using a clustering algorithm. Next, a “contamination rate” calculation was employed to rule out unrelated events nearby to each other, which left a smaller dataset of potential repeaters that were subsequently verified using localization techniques.
Including previous published research from CHIME, which saw first light in 2017, the observatory has now detected 46 confirmed repeating FRBs and perhaps as many as 60 repeaters when looking at the candidate set. CHIME, while great at examining the entire sky, is not as precise when it comes to pinpointing locations – which is where the difficulty arose in confirming all FRBs as repeating at this time.
“We really have to be careful about saying these are the same source,” Pleunis said of the suspected repeaters, and emphasized follow-up studies will be required. “Some of them, they may just happen to be close to each other – per se – but coming from different places in the universe.”
Looking at the repeaters that have been confirmed across other radio observatories, there are five that are noted as “prolific” by the authors and which – interestingly enough – come from different types of galaxies. The authors describe the quintet, mapped to subarcsecond precision, as coming from respectively ” a star-forming dwarf galaxy, a disk galaxy, another star-forming dwarf galaxy, a globular cluster, and a barred galaxy.”
FRBs have thus been found across a variety of different environments, when including those prolific repeaters and when considering other confirmed events. This situation presents difficulties for identifying the conditions under which they are formed, the authors caution, which is why they are urging more studies of FRBs where possible.
One key question that this study could narrow down, given more confirmation, would be whether repeating FRBs are distinct populations from non-repeating FRBs. Alternatively, it may be that “non-repeating” FRBs are simply slow repeaters, producing one burst a week or less that have not been detected in follow-up observations.
“What [the study] tells us is we have a we have a bunch of these infrequent repeaters,” Pleunis said. “That means that a lot may eventually repeat if you just look long enough.”