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TITLE:   GCN CIRCULAR
NUMBER:  31957
SUBJECT: GRB 220426C: Swift/BAT-GUANO detection outside the coded FOV (short)
DATE:    22/04/27 14:10:41 GMT
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/