GCN Circular 41746

E. Ambrosi  (INAF-IASFPA) , M. Capalbi (INAF-OAR), J.A. Kennea (PSU),
D.N. Burrows (PSU), S. Lanava (PSU), A.P. Beardmore (U. Leicester),
J.P. Osborne (U. Leicester), M. Perri (SSDC & INAF-OAR) and P.A. Evans
(U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of GRB 250903A. We
searched for X-ray sources in  2.4 ks of Photon Counting (PC) mode
data. The total exposure at the position of the afterglow (see below)
is 6.3 ks, obtained between T0+12.7 ks and T0+344.1 ks.

Four uncatalogued X-ray sources are detected within the estimated
3-sigma SVOM/ECLAIRs error region (509 arcsec), of which one ("Source
1") is fading with >3-sigma significance and its position is consistent
with the optical afterglow (An et al., GCN 41679
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 ). Therefore, this is
the GRB afterglow. Using 2370 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 =
356.79798, -75.96644 which is equivalent to:

RA (J2000): 23h 47m 11.52s
Dec(J2000): -75d 57' 59.2"

with an uncertainty of 3.0 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.9 (+/-0.9).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.95 (+0.73, -0.21). The
best-fitting absorption column is  consistent with the Galactic value
of 4.7 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 (3.6 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4 (+/-10) x 10^20 cm^-2
Galactic foreground: 4.7 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.95 (+0.73, -0.21)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/03000070.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/SVOM/SVOM_FIELD00034.

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