TITLE:   GCN CIRCULAR
NUMBER:  35421
SUBJECT: GRB 231222B: Swift-XRT afterglow detection
DATE:    23/12/23 12:50:08 GMT
FROM:    Phil Evans at U of Leicester <pae9@star.le.ac.uk>

M. Ferro (INAF-OAB), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto),
D.N. Burrows (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), R. Brivio (INAF-OAB), A.
D'Ai (INAF-IASFPA) and P.A. Evans (U. Leicester) reports on behalf of
the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 231222B in a series of observations tiled
on the sky. We point out that the burst was improperly named GRB
231222A in GCN Circ 35415, where the tiling was announced. The total
exposure time is 4.1 ks, distributed over 4 tiles; the maximum exposure
at a single sky location was 2.3 ks. The data were collected between
T0+43.2 ks and T0+54.8 ks, and are entirely in Photon Counting (PC)
mode. 

Three uncatalogued X-ray sources are detected, of which one ("Source
1") is believed to be the afterglow. Using 1211 s of PC mode data and 1
UVOT image, we find an enhanced XRT position (using the XRT-UVOT
alignment and matching UVOT field sources to the USNO-B1 catalogue):
RA, Dec = 251.24744, +19.62084 which is equivalent to:

RA (J2000): 16h 44m 59.39s
Dec(J2000): +19d 37' 15.0"

with an uncertainty of 3.3 arcsec (radius, 90% confidence). This
position is 9.3 arcmin from the Fermi/LAT position.  The source has a
mean count rate of 6.0e-02 ct/sec; we cannot determine at the present
time whether it is fading.

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.65 (+0.49, -0.29). The
best-fitting absorption column is  consistent with the Galactic value
of 7.1 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 3.4 x 10^-11 (3.8 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7 (+/-14) x 10^20 cm^-2
Galactic foreground: 7.1 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.65 (+0.49, -0.29)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00021639.
The results of the full analysis of the tiled XRT observations are
available at https://www.swift.ac.uk/xrt_products/TILED_GRB00118.

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