TITLE:   GCN CIRCULAR
NUMBER:  33267
SUBJECT: GRB 230204B: Swift/BAT-GUANO detection outside the coded FOV
DATE:    23/02/05 02:56:17 GMT
FROM:    Aaron Tohuvavohu at U Toronto  <aaron.tohu@gmail.com>

Jamie A. Kennea (PSU), Gayathri Raman (PSU), James DeLaunay
(UAlabama), Aaron Tohuvavohu (U Toronto) report:

Swift/BAT did not localize GRB 230204B onboard (T0:
2023-02-04T21:44:25.2 UTC, CALET trig CALET 1359582171, Fermi/GBM trig
697239872, INTEGRAL trig 10188, MAXI GCN 33265).

The CALET, INTEGRAL, and Fermi 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 21.5 in a 4.096 s
analysis time bin.
The burst duration as seen by BAT is ~65 s.

NITRATES results indicate a burst coming from outside the FOV, with
DeltaLLHOut of -4.
The OFOV localization is consistent with the MAXI position for this burst.

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/