Samuele Ronchini (PSU), Gayathri Raman (PSU), Jamie A. Kennea (PSU), James DeLaunay (UAlabama), Aaron Tohuvavohu (U Toronto), Tyler Parsotan (UMBC/GSFC) report:
Swift/BAT did not localize GRB 230418A onboard (T0: 2023-04-18T21:11:33.49 UTC, Fermi/GBM GCN 33652)
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), detects the burst with a sqrt(TS) of 31.1 in a 0.256 s analysis time bin.
The burst duration as seen by BAT is ~0.25 seconds.
NITRATES results indicate a burst coming from outside the FOV, with DeltaLLHOut of -20.
The NITRATES best fit OFOV position is consistent with the Fermi/GBM localization (GCN 33652).
See Section 9.1 and Figures 10 and 17 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/