TITLE:   GCN CIRCULAR
NUMBER:  31101
SUBJECT: GRB 211120B: Swift/BAT-GUANO detection outside the coded FOV
DATE:    21/11/21 14:44:02 GMT
FROM:    Aaron Tohuvavohu at U Toronto  <aaron.tohu@gmail.com>

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

Swift/BAT did not successfully trigger on GRB 211120B (T0: 2021-11-20
13:09:46.8 UT, Fermi Trigger: 659106591).

The Fermi 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 23.1 in a
8.192 s analysis time bin.
The estimated T90 in the detector is 24.5 s (15-350 keV).

NITRATES prefers an origin for this burst coming from outside the
coded FOV, with DeltaLLHOut of -6.

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

An out of FOV origin is consistent with the Fermi/GBM localization.

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/