TITLE: GCN CIRCULAR NUMBER: 39979 SUBJECT: GRB 250331D: Swift/BAT-GUANO arcminute localization of a burst during slew DATE: 25/03/31 17:36:04 GMT FROM: Samuele Ronchini at PSU <sjs8171@psu.edu> Samuele Ronchini (PSU), James DeLaunay (PSU), Aaron Tohuvavohu (Caltech), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report: Swift/BAT did not localize GRB 250331D onboard (T0: 2025-03-31T05:49:17 UTC, Fermi GCN 39961). The Fermi notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1). Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground. The burst occurred during a Swift slew. The imaging is performed using BatAnalysis tools (Parsotan et al. 2025 [https://arxiv.org/abs/2502.00278](https://arxiv.org/abs/2502.00278)). A confident location is found for the burst with SNR of 11.6 The BAT position is RA, Dec = 48.52543, 62.30596 deg which is RA(J2000) = 03h 14m 06.10s Dec(J2000) = +62d 18’ 21.5″ with an estimated uncertainty of 5 arcmin. This position is consistent with the Ferm/GBM localization (GCN 39961). XRT and UVOT follow-up has been requested. Results of follow-up observations will be reported in future circulars. GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches. A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/ View this GCN Circular online at https://gcn.nasa.gov/circulars/39979. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...