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GCN Circular 31925

GRB 220420A: Swift/BAT-GUANO detection outside the coded FOV
2022-04-20T15:26:11Z (2 years ago)
Aaron Tohuvavohu at U Toronto <>
Jamie A. Kennea (PSU), Gayathri Raman (PSU), Aaron Tohuvavohu (U
Toronto), James DeLaunay (UAlabama) report:

Swift/BAT did not localize GRB 220420A onboard (T0:
2022-04-20T03:54:31.9 UTC, CALET trig #1334461972, INTEGRAL/SPI-ACS
trig #9861).

The CALET 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 108 seconds of BAT event-mode data from
[-45,+63] 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 65.8 in a
16.384 s analysis time bin.
The burst episode as seen by BAT is ~60s long.

NITRATES results strongly indicate a burst coming from outside the
coded FoV, with DeltaLLHOut of -489.

See Section 9.1 and Figure 20 in the NITRATES paper for brief
descriptions and interpretation of sqrt(TS), DeltaLLHPeak, and

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:
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