GCN Circular 31957
Subject
GRB 220426C: Swift/BAT-GUANO detection outside the coded FOV (short)
Date
2022-04-27T14:10:41Z (3 years ago)
From
Aaron Tohuvavohu at U Toronto <aaron.tohu@gmail.com>
Jamie A. Kennea (PSU), Gayathri Raman (PSU), Aaron Tohuvavohu (U
Toronto), James DeLaunay (UAlabama), report:
Swift/BAT did not localize GRB 220426C onboard (T0:
2022-04-26T07:41:28.5 UTC, GECAM trig #utn220426_074128_GECAMb).
The GECAM 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,
arXiv:2111.01769), detects the burst with a sqrt(TS) of 25.5 in a
1.024 s analysis time bin.
The burst episode as seen by BAT is ~1 s long.
NITRATES results indicate a burst coming from outside the coded FoV,
with DeltaLLHOut of -9.5.
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/