CHIME tracks a hyperactive repeating fast radio burst source

Using the Canadian Hydrogen Intensity Mapping Experiment (CHIME), an international team of astronomers has performed radio observations of FRB 20220912A—a highly active source of repeating fast radio bursts. Results of the monitoring campaign, published April 10 on the preprint server arXiv, could help us better understand the nature of these enigmatic sources.

Fast radio bursts (FRBs) are intense bursts of radio emission lasting milliseconds showcasing the characteristic dispersion sweep of radio pulsars. The physical nature of these bursts is yet unknown, and astronomers consider a variety of explanations ranging from synchrotron maser emission from young magnetars in supernova remnants to cosmic string cusps.

Hyperactive repeater

FRB 20220912A is a hyperactive repeating fast radio burst discovered in 2022 with CHIME’s FRB instrument. It is located in the active galaxy PSO J347.2702+48.7066, at a redshift of 0.077. Previous observations of this source have detected burst rates of up to 390 bursts per hour and found that the estimated total emitted energy may challenge some FRB progenitor models involving magnetars.

That is why a group of astronomers led by Thomas C. Abbot of McGill University in Montreal, Canada, decided to perform follow-up observations of FRB 20220912A with CHIME’s Pulsar instrument. In general, the Pulsar instrument is designed for high-time resolution, precision pulsar observations; however, it is also suitable for in-depth studies of single repeating sources.

CHIME/Pulsar digs into FRB 20220912A

“CHIME/Pulsar produces 10 digitally formed tracking beams, which can each be centered on a source throughout its transit. This eliminates the frequency-dependent response of the static beams of CHIME/FRB and is particularly beneficial for sources that transit through regions in the CHIME/FRB beam grid that have reduced sensitivity,” the researchers explain.

“Furthermore, CHIME/Pulsar is capable of recording high-time resolution, coherently de-dispersed filterbank data, enabling 40.96 µs single pulse searches for temporally narrow FRBs from repeating sources with known dispersion measure.”

The follow-up monitoring campaign identified 828 bursts from FRB 20220912A in 201.2 hours of observations. This yields a mean burst rate of 4.12 bursts per hour at a fluence threshold of 0.92 Jy ms. The peak observed burst rate was 52 bursts per hour and it occurred October 25, 2022.

What did the Pulsar instrument find?

The observations found that the mean weekly burst rate of FRB 20220912A changes significantly with time. In particular, the source undergoes high activity in the first 10 weeks of observation and has a bimodal wait-time distribution, during which, on some individual days, the burst rate rises well above the mean. However, after the initial 10 weeks of high activity, the weekly mean burst rate is below or consistent with the overall mean burst rate.

Furthermore, the study found that the dispersion measure of FRB 20220912A increases linearly with time at a rate of 1.4 pc/cm³ per year, while its rotation measure remains near zero. This is in contrast to what has been observed for other active repeaters and suggests that FRB 20220912A may reside in a unique local environment.

The collected data also allowed the researchers to determine the total energy output for FRB 20220912A, which was estimated to be at least 10 tredecillion ergs. The authors of the study argue that extreme magnetar models invoking strong dipoles, high-order surface multipoles, or high internal fields can provide such energy.

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