GRB 210827A: Swift/BAT-GUANO detection and arcminute localization

Swift/BAT did not trigger on GRB 210827A (T0: 2021-08-27 09:59:24.34 UTC, Fermi/GBM trigger # 651751169, GCN 30724). 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 burst is detected in BAT with a duration of approximately 10 seconds. Using the normal BAT imaging technique, we find the location for the GRB with an SNR of 8.3. The BAT position is RA, Dec = 174.9181 +55.7858 deg which is RA(J2000) = 11h 39m 40.34s Dec(J2000) = +55d 47' 08.88″ with an estimated uncertainty of 4 arcmin. This position is consistent with the Fermi/GBM localization. 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/