GCN Circular 41645

J.P. Osborne (U. Leicester), T. Sbarrato (INAF-OAB), P. D'Avanzo
(INAF-OAB), M.G. Bernardini (INAF-OAB), D.N. Burrows (PSU), S. Lanava
(PSU), S. Dichiara (PSU), K.L. Page (U. Leicester) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of GRB 250901A. We
searched for X-ray sources in  754 s of Photon Counting (PC) mode data.
The total exposure at the position of the afterglow (see below) is 754
s, obtained between T0+2.6 ks and T0+3.3 ks.

An uncatalogued X-ray source is detected and is above the LSXPS 3-sigma
upper limit at this position, and is therefore likely the GRB
afterglow.  The position of this source is RA, Dec=356.1837, -15.7410
which is equivalent to:

RA (J2000): 23h 44m 44.08s
Dec(J2000): -15d 44' 27.7"

with an uncertainty of 3.9 arcsec (radius, 90% confidence). 

The light curve can be modelled with a power-law decay with a decay
index of alpha=3.2 (+1.8, -3.0).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.9 (+0.5, -0.4). The
best-fitting absorption column is  6 (+12, -4) x 10^20 cm^-2,
consistent with the Galactic value of 1.9 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 3.5 x 10^-11 (4.0 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6 (+12, -4) x 10^20 cm^-2
Galactic foreground: 1.9 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.9 (+0.5, -0.4)

If the light curve continues to decay with a power-law decay index of
3.2, the count rate at T+24 hours will be 2.9 x 10^-6 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.0 x
10^-16 (1.1 x 10^-16) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/03000066.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/SVOM_FIELD00033.

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