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GCN Circular 33499

Subject
GRB 230320B: Swift/BAT-GUANO detection outside the coded FOV
Date
2023-03-21T23:43:53Z (a year ago)
From
Aaron Tohuvavohu at U Toronto <aaron.tohu@gmail.com>
Samuele Ronchini (PSU), Gayathri Raman (PSU), Jamie A. Kennea (PSU),
James DeLaunay (UAlabama), Aaron Tohuvavohu (U Toronto) report:

Swift/BAT did not localize GRB 230320B onboard (T0:
2023-03-20T21:12:25 UTC, Fermi/GBM trig 701039550, CALET trig
1363381768, GECAM trig 163, AstroSat/CZTI GCN 33492).

The Fermi, CALET and GECAM 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 2022, ApJ,
941, 169), detects the burst with a sqrt(TS) of 12.04 in a 4.096 s
analysis time bin.
The Fermi/GBM localization is consistent with being outside the BAT FOV.

See Section 9.1 and Figures 10 and 17 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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