GCN Circular 41733

M.G. Bernardini (INAF-OAB), E. Ambrosi	(INAF-IASFPA) , M. Capalbi
(INAF-OAR), S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU),
K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 4.7 ks of XRT data for GRB 250905A, from 140 s to 27.9
ks after the   trigger. The data comprise 26 s in Windowed Timing (WT)
mode with the remainder in Photon Counting (PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=5.2 (+0.4, -0.3).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.9 (+0.8, -0.6). The
best-fitting absorption column is  1.8 (+3.2, -1.2) x 10^21 cm^-2,
consistent with the Galactic value of 5.5 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 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:	     1.8 (+3.2, -1.2) x 10^21 cm^-2
Galactic foreground: 5.5 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.9 (+0.8, -0.6)

If the light curve continues to decay with a power-law decay index of
5.2, the count rate at T+24 hours will be 2.5 x 10^-14 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.0 x
10^-24 (1.3 x 10^-24) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01347265.

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