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GCN Circular 31954

Subject
GRB 220426A: Swift/BAT-GUANO detection outside the coded FOV
Date
2022-04-26T19:00:41Z (2 years ago)
From
Aaron Tohuvavohu at U Toronto <aaron.tohu@gmail.com>
Aaron Tohuvavohu (U Toronto), Gayathri Raman (PSU),  James DeLaunay
(UAlabama), Jamie A. Kennea (PSU), report:

Swift/BAT did not localize GRB 220426A onboard (T0:
2022-04-26T06:49:51 UTC, Fermi/GBM trig #672648596, INTEGRAL/SPI-ACS
trig #9876).

The Fermi and INTEGRAL 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 169 in a 4.096
s analysis time bin.
The burst episode as seen by BAT is ~6s long.

NITRATES results strongly indicate a burst coming from outside the
coded FoV, with DeltaLLHOut of -1550, consistent with the Fermi/GBM
BALROG localization (GCN 31950).

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/
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