CHIME/FRB and Outriggers localization of Repeating FRB 20251229A

ATel #17709; Lordrick Kahinga (University of California, Santa Cruz) on behalf of the CHIME/FRB Collaboration
on 5 Mar 2026; 16:56 UT
Credential Certification: Kaitlyn Shin (kshin@mit.edu)

Tweet

In ATels #17574 and #17606, we reported the discovery and updated arcminute-scale localization of a potentially hyperactive fast radio burst (FRB) source, FRB 20251229A, using the CHIME telescope. We have now used CHIME/FRB Outriggers data, captured for the second of the three reported bursts (recorded on 2026-01-01 UT 21:56:03), to localize the burst with CHIME core, the Green Bank Outrigger (GBO), and the KKO Outrigger. We note that the Hat Creek Outrigger was undergoing commissioning work during this time, and did not record data for any of the three reported bursts. Using the delay calibrators J2035+1857 on the GBO-CHIME baseline and J2045+3233 on the CHIME-KKO baseline, we report a localization ellipse in the ICRF of: RA: 20h 41m 23.82s DEC: +16° 01' 51.34" a_error (semi-major axis): 20” b_error (semi-minor axis): 0.07” theta: 13.2 degrees east of North This localization falls within the Pan-STARRS footprint, which has a 5-sigma depth of r ~ 23.0 mag (Chambers, K. C., et al. 2016). To determine the most likely host galaxy for FRB 20251229A, we use PATH (Aggarwal et al. 2021) to assign probabilities of host association, P(O|x), to each galaxy in the field. We find the most likely host galaxy to be J204123.23+160126.54 (i.e. G1; r = 18.6 mag), which lies 26.2" from the center of the localization ellipse and has a photometric redshift of 0.08 from PS1-WISE-STRM (Beck et al. 2021). We note the presence of an additional faint source J204124.38+160222.58 (i.e. S1; r = 21.1 mag), located 32.3" from the center of the ellipse but it is likely a star based on multiple star-galaxy classifiers (Tachibana & Miller 2018; Beck et al. 2021), its unresolved r-band light profile, and a proper motion measurement in the extended Gaia-PS1-SDSS proper motion catalog. Conservatively, if we consider S1 to be a galaxy, and that the probability that the galaxy is unseen in the relatively shallow Pan-STARRS imaging has a prior of P(U) = 0.32 (Andersen et al. in prep), we find the probability of association of G1 to be P(O|x) = 0.732. However, if we consider S1 to be a star, then the P(O|x) of G1 increases to 0.834 with a posterior that the host is unseen P(U|x) of 0.12. With present data, we find G1 to be the most likely host. However, we note that we cannot rule out the possibility that the actual host galaxy exists within the localization ellipse below the detection threshold of PS1. Finally, within the localization ellipse or at the position of G1, no radio source is detected in VLASS cutouts, which achieve a one-sigma RMS of approximately 90 uJy per beam in the 2-4 GHz band. We encourage further follow-up of this field to improve the positional precision of the FRB and help to solidify the host galaxy association. Attached is the Pan-STARRS image of the FRB 20251229A localization region. The cyan cross marks the center of the localization, with the cyan ellipse indicating the 3 sigma error ellipse. The yellow and white squares mark the most likely galaxy candidate and potential stellar contaminant, respectively. G1 (J204123.23+160126.54, r = 18.6 mag) is the most probable host galaxy, with PATH posterior probability P(O|x) = 0.72. This probability increases to 0.83 upon exclusion of the likely star S1 (J204124.38+160222.58, r = 21.1 mag).

PATH Results image