TITLE: GCN CIRCULAR NUMBER: 30134 SUBJECT: GRB 210606A: Swift/BAT-GUANO detection and arcminute localization DATE: 21/06/06 17:43:04 GMT FROM: Aaron Tohuvavohu at U Toronto James DeLaunay (PSU), Aaron Tohuvavohu (U Toronto), Jamie A. Kennea (PSU), Gayathri Raman (PSU) report: Swift/BAT did not trigger on GRB 210606A (T0: 2021-06-06 03:56:02 UTC, Fermi/GBM GCN 30131). 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 ~8 seconds. With a maximum likelihood analysis (DeLaunay et al. 2021, in prep.) on the event-mode data we detect a location for the burst with a square root of the test statistic, sqrt(TS), of 15. The sqrt(TS) behaves similarly to SNR. Using the normal BAT imaging technique, we find the same location for the GRB with an SNR of 5.2. The BAT position is RA, Dec = 170.904, +0.718 deg which is RA(J2000) = 11h 23m 36.96s Dec(J2000) = 00d 43′ 04.8″ with an estimated uncertainty of 4 arcmin. The partial coding was 8%. This position is consistent with the Ferm/GBM localization (GCN 30131). 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/ [GCN OPS NOTE(06Jun21): Per author's request, the two places that cited GCN 31031 have been changed to GCN 30131.]