TITLE: GCN CIRCULAR NUMBER: 32148 SUBJECT: GRB 220528B: Swift/BAT-GUANO detection outside the coded FOV DATE: 22/05/28 17:32:30 GMT FROM: Aaron Tohuvavohu at U Toronto Gayathri Raman (PSU), Aaron Tohuvavohu (U Toronto), James DeLaunay (UAlabama), Jamie A. Kennea (PSU), report: Swift/BAT did not localize GRB 220528B onboard (T0: 2022-05-28T06:12:47 UTC, Fermi/GBM GCN 32147). 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, arXiv:2111.01769), detects the burst with a sqrt(TS) of 12.2 in a 0.512 s analysis time bin. The burst episode as seen by BAT is ~1 s long. NITRATES results are consistent with a burst coming from outside of the coded FoV, as indicated by the Fermi/GBM localization (GCN 32147). See Section 9.1 and Figure 20 in the NITRATES paper for brief descriptions and interpretation of sqrt(TS), DeltaLLHPeak, and DeltaLLHOut. 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/