TITLE: GCN CIRCULAR NUMBER: 43334 SUBJECT: FRB 20251229A: A potentially hyperactive repeating fast radio burst source DATE: 26/01/05 17:53:25 GMT FROM: nicole.mulyk@mail.mcgill.ca Nicole Mulyk (McGill University, Trottier Space Institute) on behalf of the CHIME/FRB Collaboration: The CHIME/FRB Collaboration reports the detection of three bursts from a new repeating fast radio burst (FRB) source over the last week, with TNS designation FRB 20251229A. These bursts were detected on 2025-12-29 22:00:58, 2026-01-01 21:56:03, and 2026-01-03 21:53:36 UTC (400 MHz, topocentric). The first two detections were streamed through our VOEvents service. The third burst did not trigger a VOEvents notification as its signal-to-noise ratio was below the trigger threshold of 12 (see table below; Abbott et al. 2025). The rapid succession of these bursts in combination with the low declination of the source (+16 degrees; see below), and low CHIME exposure of only ~5 minutes per day (see note below), provides evidence of high activity. Additionally, this FRB source has a relatively low DM of 192.145 +/- 0.013 pc/cm^3, calculated by structure-maximizing the time profile using the highest signal-to-noise burst (Seymour, Michilli, & Pleunis 2019). The expected Galactic contribution along this line of sight is 93.6 or 75.3 pc/cm^3 from NE2001 (Cordes & Lazio 2002) or YMW16 (Yao, Manchester & Wang 2017), respectively. The large disparity between the predicted maximum Galactic DM from both models, and the much larger source DM, strongly suggests this is an extragalactic source. Assuming an average contribution of the intergalactic medium to the DM (Macquart et al. 2020), we expect the FRB’s redshift to be within ~0.1. Initial analysis of the raw voltage data of the brightest event from 2026-01-01 provides a best-fit localization, of RA (J2000): 20h41m26.3s +/- 1.2s, Dec (J2000): 16°01’24” +/- 18”, with errors quoted at 1-sigma uncertainty (Michilli et al. 2021). Using the raw voltage data for the 2026-01-01 burst, we obtain a peak flux of 30 +/- 3 Jy and a 400 to 800 MHz band-averaged fluence of 160 +/- 16 Jy ms (CHIME/FRB Collaboration 2024). We have attached the waterfall plot for the 2026-01-01 burst in the link at the end of this ATel. Due to the potentially high repetition rate and relative proximity of the source, we encourage multi-wavelength follow-up. Note: This is compared to a “typical” daily exposure of ~15 minutes near zenith (see also Fig. 5 of CHIME/FRB Collaboration 2021, ApJS, 257, 59). Topocentric Burst 400 MHz Arrival Time (UTC) | Detection Signal to Noise (S/N) Ratio : 2025-12-29 22:00:58 | 13.36 2026-01-01 21:56:03 | 47.90 2026-01-03 21:53:36 | 11.25 Waterfall plot: https://storage.googleapis.com/chimefrb-dev.appspot.com/FRB20251229A/FRB2025... View this GCN Circular online at https://gcn.nasa.gov/circulars/43334. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...