TITLE: GCN CIRCULAR NUMBER: 30916 SUBJECT: SGR 1935+2154: Swift/BAT-GUANO detection and arcminute localization DATE: 21/10/01 18:01:34 GMT FROM: Aaron Tohuvavohu at U Toronto <aaron.tohu@gmail.com> Aaron Tohuvavohu (U Toronto), James DeLaunay (PSU), Gayathri Raman (PSU), Jamie A. Kennea (PSU) report: Swift/BAT did not localize SGR 1935+2154 (T0: 2021-10-01 00:04:04.3 UTC, Fermi/GBM trigger # 654739449) onboard. The Fermi/GBM 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, in prep), detects a burst and localizes it to the position of SGR 1935+2154. The burst is detected in BAT with a duration of less than 64 ms. This position is consistent with the Ferm/GBM localization. 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/