TITLE: GCN CIRCULAR NUMBER: 31006 SUBJECT: GRB 211024A: Swift/BAT-GUANO detection and candidate arcminute localization of a short burst DATE: 21/10/26 13:53:49 GMT FROM: Aaron Tohuvavohu at U Toronto <aaron.tohu@gmail.com> Aaron Tohuvavohu (U Toronto), James DeLaunay (UAlabama), Gayathri Raman (PSU), Jamie A. Kennea (PSU), report: Swift/BAT did not trigger on GRB 211024A (T0: 2021-10-24T01:34:11.4 UTC, Fermi/GBM trigger # 656732056, GCN 30962), also detected by GECAM-B (GCN 30974). 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), confidently detects the burst with a duration of ~0.1 seconds and sqrt(TS) of 10.4. A candidate localization is found with DeltaLLHOut of 13 and an ambiguous DeltaLLHPeak of 2.5. For this reason we can confidently claim detection, and some preference for an in-FOV origin, but the best fit arcmin localization is not strongly preferred over other positions. Conventional imaging does not find a location for this burst. The best BAT position is RA, Dec = 28.053 -6.973 deg which is RA(J2000) = 1h 52m 12.61s Dec(J2000) = -6d 58′ 23″ with an estimated uncertainty of 6 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/