CHIME/Slow overview and pilot survey: A new backend to search for second-duration radio transients with the CHIME telescope

TL;DR

CHIME/Slow introduces a real-time, multi-scale backend to search for second-duration radio transients with CHIME, using a compressed ~3-bit intensity stream downsampled to $16\,\mathrm{ms}$, $128\,\mathrm{ms}$, and $512\,\mathrm{ms}$ and analyzed with PRESTO, HDBSCAN, and FETCH. In a pilot study on archival data, nine bursts were detected (eight from FRB 20220912A and one new FRB 20230204C); burst modeling and redshift inferences were used to derive an all-sky rate of $184$–$4556$ sky$^{-1}$ day$^{-1}$ for pulses with widths in $[16\,\mathrm{ms},5\,\mathrm{s}]$ at 600 MHz and fluence > 5 Jy ms. An injection-based completeness analysis quantifies sensitivity and current limitations, guiding future improvements in real-time performance, RFI handling, and classification. Overall, CHIME/Slow aims to fill the largely unexplored second-duration transient gap and inform the nature of these events.

Abstract

We present an overview of CHIME/Slow, a real-time transient search backend under development to search for second-duration radio transients using the CHIME telescope, and results obtained from a pilot survey carried out using the prototype version of the search pipeline. The prototype CHIME/Slow pipeline was tested on archival data obtained in December 2022, January 2023 and February 2023 with a total on-sky time of 17 days with an instantaneous Field of View (FoV) of $\sim$13 deg$^2$ . In this pilot survey, we detected nine bursts, one from a new non-repeating source and eight from the known hyperactive repeating source FRB 20220912A. Out of these nine bursts, two bursts from the repeater were not detected by CHIME/FRB, while the non-repeater was detected in the side-lobe of a beam in the CHIME/FRB exhibiting shorter pulse width and narrower bandwidth compared to the CHIME/Slow detection. Here we report properties of the bursts, discuss the sensitivity and completeness of the current version of the CHIME/Slow pipeline, and outline future development to improve its performance. Finally, based on these results, we report the all-sky rate (95% credible region) of radio transients with pulse widths between 16 ms to 5 s, fluence above 5 Jy ms and observing frequency of 600 MHz to be between 184 and 4556 bursts sky$^{-1}$ day$^{-1}$.

Figures (3)

• Figure 1: Radio transients associated with compact objects in the pseudo-luminosity vs. pseudo pulse width parameter space. The parts of the parameter space covered by CHIME/FRB and CHIME/Slow are highlighted in light gray and light yellow colors, respectively. The green points near the FRB population show the CHIME/Slow detections from the pilot survey with the circles indicating bursts from FRB 20220912A and the triangle indicating the non-repeating burst FRB 20230204C (See Section \ref{['sec:survey']} for more details). Figure adapted from Cordes2004 and Keane2018 with data from Hurley-Walker2022Hurley-Walker2023Caleb2022Caleb2024Dong2024de_Ruiter2024 and this work. Note that the shaded regions only mark the widths covered.
• Figure 2: Overview of the CHIME/Slow pipeline: The diagram shows the major steps in the pipeline and the tools used to perform these steps.
• Figure 3: Dynamic spectra ("waterfall" plots), frequency-averaged time series, and time-averaged spectra of bursts detected in the CHIME/Slow pilot survey. The bursts are plotted in order of their arrival times (topocentric) from top-left to bottom-right. Each plot is corrected for the best-fit DM obtained from fitburst. The blue lines in the time-profile and spectrum plot show the best-fit frequency-averaged model for the burst profile and time-averaged spectral model estimated over the burst duration respectively. The last burst, inside the blue box, is the one-off event FRB 20230204C, while all other bursts are from the repeating source FRB 20220912A. Out of these eight bursts, the two marked by red boxes were not detected by CHIME/FRB, while the other six were co-detections.