GCN Circular 42782

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M.G. Bernardini
(INAF-OAB), E. Ambrosi	(INAF-IASFPA) , M. Capalbi (INAF-OAR), D.N.
Burrows (PSU), S. Lanava (PSU), S. Dichiara (PSU) 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 251118A. We searched for X-ray sources in 
3.0 ks of Photon Counting (PC) mode data. The total exposure at the
position of the afterglow (see below) is 5.4 ks, obtained between
T0+31.3 ks and T0+108.8 ks.

Two uncatalogued X-ray sources are detected, of which one ("Source 1")
is above the RASS 3-sigma upper limit at this position, and is
therefore likely the GRB afterglow. Using 3939 s of PC mode data and 7
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 = 160.00418, -34.52259 which is equivalent to:

RA (J2000): 10h 40m 01.00s
Dec(J2000): -34d 31' 21.3"

with an uncertainty of 2.0 arcsec (radius, 90% confidence). This
position is 5.0 arcmin from the Fermi/LAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.08 (+0.18, -0.16).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.73 (+0.16, -0.15). The
best-fitting absorption column is  2.2 (+0.7, -0.6) x 10^21 cm^-2, in
excess of the Galactic value of 8.3 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 4.1 x 10^-11 (5.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     2.2 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 8.3 x 10^20 cm^-2
Excess significance: 3.9 sigma
Photon index:	     1.73 (+0.16, -0.15)

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/00021890.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021890.

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