TITLE:   GCN CIRCULAR
NUMBER:  45755
SUBJECT: GRB 260925A: Swift-XRT refined Analysis
DATE:    26/09/26 07:34:22 GMT
FROM:    Phil Evans at U of Leicester <pae9@star.le.ac.uk>

J.A. Kennea (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U.
Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), A. D'Ai
(INAF-IASFPA), D.N. Burrows (PSU), S. Lanava (PSU) and P.A. Evans
report on behalf of the Swift-XRT team:

We have analysed 4.2 ks of XRT data for GRB 260925A, from 119 s to 28.6
ks after the   trigger. The data are entirely in Photon Counting (PC)
mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.95 (+0.30, -0.25).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.6 (+0.5, -0.4). The
best-fitting absorption column is  9 (+24, -6) x 10^20 cm^-2,
consistent with the Galactic value of 3.6 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 4.3 x 10^-11 (4.8 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9 (+24, -6) x 10^20 cm^-2
Galactic foreground: 3.6 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.6 (+0.5, -0.4)

If the light curve continues to decay with a power-law decay index of
2.95, the count rate at T+24 hours will be 8.9 x 10^-9 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.8 x
10^-19 (4.3 x 10^-19) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01513316.

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