James DeLaunay (U Alabama, PSU), Aaron Tohuvavohu (U Toronto), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report:
Swift/BAT did not localize GRB 231017A onboard (T0: 2023-10-17T08:05:03.30 UTC, Fermi/GBM GCN 34822, Trigger 719222708).
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 2022, ApJ, 941, 169), performed on the temporal window [T0-20 s, T0+20 s], detects the burst with a sqrt(TS) of 15 in a 2.048 s analysis time bin, starting at T0 - 0.512 s.
A candidate arcminute localization is found with DeltaLLHOut of 17 and a DeltaLLHPeak of 6.
See Section 9.1 and Figures 10 and 17 in the NITRATES paper for brief descriptions and interpretations of sqrt(TS), DeltaLLHPeak, and DeltaLLHOut.
The BAT position is
RA, Dec = 86.191, +56.666 deg which is
RA(J2000) = 05 44m 45.84s
Dec(J2000) = +56d 39′ 57.6″
with an estimated uncertainty of 6 arcmin radius.
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/