TITLE:   GCN CIRCULAR
NUMBER:  31158
SUBJECT: GRB 211201A: Swift/BAT-GUANO detection likely outside the coded FOV
DATE:    21/12/02 21:06:16 GMT
FROM:    Aaron Tohuvavohu at U Toronto  <aaron.tohu@gmail.com>

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

Swift/BAT did not trigger on GRB 211201A (T0: 2021-12-01T20:42:52 UTC,
Fermi/GBM GCN 31155).

The Fermi/GBM 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 13.1 in a
16.384 s analysis time bin.
Estimated T90 in the detector is 30.5 +/- 0.5 s (15-350 keV).

NITRATES results, independently, are ambiguous with respect to whether
this burst originates from in or outside the BAT FOV, with a
borderline DeltaLLHOut of 9.57.
An out of FOV origin is more consistent with the Fermi/GBM
localization, and the Fermi/GBM spectral fit (private comm.) is
consistent with the best fit OFOV model from NITRATES.
Therefore we think it likely that this burst originates from outside
the coded FOV.

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/