TITLE: GCN CIRCULAR NUMBER: 39886 SUBJECT: FRB 20250316A: The sub-arcsecond localization of FRB 20250316A using CHIME/FRB Outriggers coincides with reported X-ray counterparts DATE: 25/03/27 21:19:58 GMT FROM: shiona@mit.edu We have derived a sub-arcsecond VLBI localization for FRB 20250316A (see ATel #17081, ATel #17086; this is the first such localization using the full CHIME/FRB Outriggers array. This consists of the CHIME core and the KKO Outrigger (Lanman et al. 2024) near Penticton, BC, and two additional Outrigger stations at Green Bank Observatory (GBO) and Hat Creek Radio Observatory (HCO). The position below is consistent with the ~10" position of the potential X-ray counterpart EP J120944.2+585060 detected by the Einstein Probe (ATel #17100) but is inconsistent with the 90% confidence interval reported from Swift/XRT observations (ATel #17109). We derive our position by referencing delays on each of the three CHIME-Outrigger baselines to nearby in-beam International Celestial Reference Frame (ICRF) calibrators observed simultaneously with the FRB (Andrew et al. 2024). On all CHIME-Outrigger baselines, we phase reference our visibilities to the calibrator RFC J1204+5202 (https://astrogeo.org/sol/rfc/rfc_2024b/; Petrov & Kovalev 2025), located ~6 degrees away from the FRB. Five additional in-beam ICRF calibrators were observed on all three baselines between CHIME and its Outriggers, which we use to derive a representative error budget for the FRB localization. The calibrators completely surround the FRB, spanning ~+/- 1 degree in hour angle and +50/-20 degrees in declination relative to CHIME zenith, while the FRB is located ~0.2 degrees east and ~10 degrees north of CHIME zenith. We check for imperfect calibrator selection and direction-dependent effects by 1) calibrating the target to all other usable calibrators, and 2) calibrating known calibrator positions to other calibrators, which maximizes on-sky angular separations. Both procedures give RMS delay errors no larger than 1 ns, 3 ns, and 6ns on the CHIME-KKO, CHIME-HCO and CHIME-GBO baselines, respectively. This yields the following 1-sigma (stat+sys) localization ellipse for the location of FRB 20250316A: Right Ascension = 12h09m44.31s Declination = +58d50m56.70s a_err = 150 milliarcsec b_err = 100 milliarcsec theta = 2.4 degrees East of North All coordinates are in the ICRF. We have included our one-sigma localization contour over the Epoch 1 r-band MMT image reported in ATel #17112. We note that to account for astrometric offsets due to optical-radio reference frame ties, we have inflated our localization errors in the plot by 150mas (added in quadrature). The full four-station array is still being commissioned, and its astrometric performance will be fully characterized in an upcoming work. However, we note that the error from our bootstrapping procedure is consistent with our archival test localizations of over 200 ICRF calibrators and well-localized pulsars at similar target-calibrator separations and at cross-correlation signal-to-noise ratios lower than those in the dataset used here. Furthermore, the abundance of in-beam calibrators may enable additional improvements on the preliminary position quoted here. Follow-up observations are strongly encouraged to definitively establish whether EP J120944.2+585060 is spatially coincident with FRB 20250316A, and to characterize the nature of this potential X-ray counterpart. Link to MMT Image https://storage.googleapis.com/chimefrb-dev.appspot.com/FRB20250316A/FRB2025... View this GCN Circular online at https://gcn.nasa.gov/circulars/39886. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...