CHIME Detects Even More Repeating Bursts

In October of 2018, we wrote about a new project to study fast radio bursts (FRBs) — brief, energetic flashes of light from beyond our galaxy. At the time, we knew of about 30 FRB sources; the new project by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope in British Columbia promised to dramatically increase that number.

Now, a year and a half later, we can see the impressive progress made: CHIME has already detected around 700 bursts from FRB sources! Included among those is the collaboration’s latest announcement: nine new repeating sources.

A Question of Repetition

FRBs were first discovered more than a decade ago. These bright, short (around a millisecond) flashes of radio emission are a million times brighter than the brightest pulses from galactic pulsars, and they carry the signature of being produced at a great distance — something that has been further confirmed by the localization of several FRBs to faraway galaxies.

Despite all we’ve learned about FRBs, we still don’t know how they’re produced — though the list of theories has now grown large enough that there’s actually a living catalog of them. One particular puzzlement is that some FRBs have been observed to repeat, whereas others have produced only one detected flash.

Does this mean that the two types of FRBs — repeating and non-repeating — are produced in two different ways? Or in two different environments? Or is there another explanation for why some repeat and others don’t?

Clues from New Flashes

To answer these questions, our best bet is to find enough FRBs to be able to make statistical inferences — and CHIME is helping to build a large sample. In a new publication led by Emmanuel Fonseca (McGill University, Canada), the CHIME collaboration presents a collection of bursts from nine new repeating FRB sources, bringing the total number of known repeaters to 20.

What does this new sample tell us? So far, it’s confirmed previous assessments of the two populations of repeaters and non-repeaters:

1. The dispersion measures — a measure of the matter the signals travel through to reach us — for repeaters have the same distribution as those for non-repeaters, suggesting the two populations originate in similar local environments and have similar distributions in space.
2. The pulse widths are larger for repeaters than for non-repeaters, meaning that repeating sources have slightly longer-duration bursts. This may point to different emission mechanisms for the two types of bursts.
3. The Faraday rotation measures — a measure of the magnetized environment around the burst source — were obtained for two of the new repeaters, and they are lower than the surprisingly high rotation measure of FRB 121102, the first known repeater. We don’t have enough measurements to tell for certain yet, but it’s starting to look like FRB 121102 is an anomaly, and both repeaters and non-repeaters typically originate from more modestly magnetized environments.

We still have a lot to figure out, but as we build up FRB statistics with samples like these, we can start to rule out some of the many origin theories for fast radio bursts. It’s exciting to watch this field as it rapidly evolves!

Citation

“Nine New Repeating Fast Radio Burst Sources from CHIME/FRB,” E. Fonseca et al 2020 ApJL 891 L6. doi:10.3847/2041-8213/ab7208