TITLE: GCN CIRCULAR NUMBER: 40563 SUBJECT: GRB 250527C: Swift/BAT-GUANO arcminute localization of a burst DATE: 25/05/28 15:52:17 GMT FROM: Jimmy DeLaunay at Penn State <delauj2@gmail.com> James DeLaunay (PSU), Aaron Tohuvavohu (Caltech), Samuele Ronchini (PSU), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report: Swift/BAT did not localize GRB 250527C onboard (T0: 2025-05-27T13:58:32.41 UTC, Fermi Trig 770047117). 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 BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), detects the burst in a 8.192 s analysis time bin starting at T0 - 4.096 s with a sqrt(TS) of 11.6. An arcminute localization is found with DeltaLLHOut of 29.5 and a DeltaLLHPeak of 17.4. See Section 9.1 and Figures 10 and 17 in the NITRATES paper for brief descriptions and interpretations of sqrt(TS), DeltaLLHPeak, and DeltaLLHOut. The BAT position is RA, Dec = 319.633, 52.737 deg which is RA(J2000) = 21h 18m 31.92s Dec(J2000) = 52d 44’ 13.2″ with an estimated uncertainty of 5 arcmin radius. More details about this burst can be found on the trigger report page here: https://guano.swift.psu.edu/trigger_report?id=770047148 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/40563. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...