GCN Circular 34587
Subject
GRB 230827A: Swift/BAT-GUANO detection of a burst outside the coded FOV
Date
2023-08-28T19:25:30Z (8 months ago)
From
Jimmy DeLaunay at University of Alabama <delauj2@gmail.com>
Via
Web form
James DeLaunay (U Alabama), Aaron Tohuvavohu (U Toronto), Samuele Ronchini (PSU), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report:
Swift/BAT did not localize GRB 230827A onboard (T0: 2023-08-27T18:17:52.93 UTC, Fermi trig 714853077)
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 90 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 54.6 in a 8.192 s analysis time bin, starting at T0 + 18.432 s.
NITRATES results are consistent with a burst coming from outside the FOV, with DeltaLLHOut of -61 and are consistent with Fermi GBM's localization (GCN 34575).
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/