GCN Circular 31934
Subject
GRB 220421A: Swift/BAT-GUANO detection outside the coded FOV
Date
2022-04-21T21:25:41Z (3 years ago)
From
Aaron Tohuvavohu at U Toronto <aaron.tohu@gmail.com>
Gayathri Raman (PSU), Aaron Tohuvavohu (U Toronto), James DeLaunay
(UAlabama), Jamie A. Kennea (PSU), report:
Swift/BAT did not localize GRB 220421A onboard (T0:
2022-04-21T08:17:54.5 UTC, CALET trig #1334564169, Fermi/GBM trig
#672221879).
The CALET and Fermi notices, 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,
arXiv:2111.01769), detects the burst with a sqrt(TS) of 34.7 in a
4.096 s analysis time bin.
The burst episode as seen by BAT is ~10s long.
NITRATES results indicate a burst coming from outside the coded FoV,
with DeltaLLHOut of -25. An out of FOV origin is consistent with the
Fermi/GBM localization.
See Section 9.1 and Figure 20 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/