GCN Circular 41719

J.P. Osborne (U. Leicester), P. D'Avanzo (INAF-OAB), M.G. Bernardini
(INAF-OAB), E. Ambrosi	(INAF-IASFPA) , S. Lanava (PSU), S. Dichiara
(PSU), J.A. Kennea (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 7.0 ks of XRT data for GRB 250904A, from 66 s to 45.6
ks after the   trigger. The data comprise 9 s in Windowed Timing (WT)
mode (taken while Swift was slewing), 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=0.85 (+0.05, -0.04).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.05 (+0.27, -0.25). The
best-fitting absorption column is  4.4 (+1.4, -1.0) x 10^21 cm^-2,
consistent with the Galactic value of 3.3 x 10^21 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 4.0 x 10^-11 (6.5 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.4 (+1.4, -1.0) x 10^21 cm^-2
Galactic foreground: 3.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.05 (+0.27, -0.25)

If the light curve continues to decay with a power-law decay index of
0.85, the count rate at T+24 hours will be 7.0 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.8 x
10^-13 (4.5 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01347179.

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