TITLE:   GCN CIRCULAR
NUMBER:  33139
SUBJECT: GRB 221231A: Swift-XRT afterglow detection
DATE:    23/01/02 11:54:58 GMT
FROM:    Phil Evans at U of Leicester  <pae9@leicester.ac.uk>

M. Perri (SSDC & INAF-OAR), A. Tohuvavohu (U. Toronto), D.N. Burrows
(PSU), J. D. Gropp (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), V. D'Elia (SSDC & INAF-OAR),
B. Sbarufatti (INAF-OAB) and P.A. Evans (U. Leicester) report on behalf
of the Swift-XRT team:

Swift-XRT has conducted further observations of the field of the
Swift/BAT-GUANO-detected burst GRB 221231A. The observations now extend
from T0+32.7 ks to T0+120.0 ks. 

Of the sources reported by Perri et al. (GCN Circ. 33135), "Source 2"
is fading with >3-sigma significance, and is therefore likely the GRB
afterglow. Using 4154 s of PC mode data and 2 UVOT images, we find an
enhanced XRT position (using the XRT-UVOT alignment and matching UVOT
field sources to the USNO-B1 catalogue): RA, Dec = 336.25600, +25.12978
which is equivalent to:

RA (J2000): 22h 25m 01.44s
Dec(J2000): +25d 07' 47.2"

with an uncertainty of 3.0 arcsec (radius, 90% confidence). This
position is 34 arcsec from the Swift/BAT-GUANO position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.1 (+0.6, -0.5).

The results of the XRT-team automatic analysis of the likely afterglow
are at https://www.swift.ac.uk/ToO_GRBs/00021534/Source2.php.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021534.

This circular is an official product of the Swift-XRT team.