TITLE:   GCN CIRCULAR
NUMBER:  33132
SUBJECT: GRB 221231A: Swift/BAT-GUANO arcminute localization of a possibly short burst
DATE:    23/01/01 06:08:19 GMT
FROM:    Aaron Tohuvavohu at U Toronto  <aaron.tohu@gmail.com>

Swift/BAT did not localize GRB 221231A onboard (T0:
2022-12-31T21:46:05 UTC, Fermi/GBM GCN 33129).

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 in a 0.256 s analysis time bin
with a sqrt(TS) of 14.
An arcminute localization is found with DeltaLLHOut of 18.2 and a
DeltaLLHPeak of 7.9.

The burst duration as seen by BAT is less than 0.5 seconds.

See Section 9.1 and Figures 10 and 17 in the NITRATES paper for brief
descriptions and interpretations of sqrt(TS), DeltaLLHPeak, and
DeltaLLHOut.

The BAT position is
RA, Dec = 336.260, +25.138 deg which is
   RA(J2000)  = 22h 25m 02.47s
   Dec(J2000) = +25d 08′ 18.6″
with an estimated uncertainty of 5 arcmin radius.

XRT and UVOT follow-up has been requested.
Results of follow-up observations will be reported in future circulars.

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/