GCN Circular 38849

A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), A. Melandri
(INAF-OAR), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea
(PSU), D.N. Burrows (PSU), M. A. Williams (PSU) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has conducted further observations of the field of the
SVOM/ECLAIRs-detected burst GRB 250103A. The observations now extend
from T0+24.3 ks to T0+432.8 ks and have a total exposure time of 5.5
ks. The source previously reported, "Source 1", is fading with >3-sigma
significance, and is therefore likely the GRB afterglow. Using 1509 s
of PC mode data and 3 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 = 22.06415, -5.05179 which is equivalent
to:

RA (J2000): 01h 28m 15.40s
Dec(J2000): 05d 03' 06.4"

with an uncertainty of 2.5 arcsec (radius, 90% confidence). This
position is 2.9 arcmin from the SVOM/ECLAIRs position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.82 (+0.24, -0.18).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.14 (+0.31, -0.28). The
best-fitting absorption column is  1.4 (+0.9, -0.7) x 10^21 cm^-2, in
excess of the Galactic value of 4.5 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 3.2 x 10^-11 (4.4 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.4 (+0.9, -0.7) x 10^21 cm^-2
Galactic foreground: 4.5 x 10^20 cm^-2
Excess significance: 2.3 sigma
Photon index:	     2.14 (+0.31, -0.28)

The results of the XRT-team automatic analysis of the likely afterglow
are at https://www.swift.ac.uk/ToO_GRBs/00021752/Source1.php.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021752.

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