TITLE:   GCN CIRCULAR
NUMBER:  42017
SUBJECT: GRB 250925A: Swift-XRT refined Analysis
DATE:    25/09/27 18:53:06 GMT
FROM:    Phil Evans at U of Leicester <pae9@star.le.ac.uk>

A. Melandri (INAF-OAR), S. Lanava (PSU), S. Dichiara (PSU), J.A. Kennea
(PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P.
Beardmore (U. Leicester), T. Sbarrato (INAF-OAB), P. D'Avanzo
(INAF-OAB) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 7.8 ks of XRT data for GRB 250925A, from 3.7 ks to
172.4 ks after the   trigger. The data are entirely in Photon Counting
(PC) mode. Using 4854 s of PC mode data and 10 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 = 341.21377, +46.55568
which is equivalent to:

RA (J2000): 22h 44m 51.31s
Dec(J2000): +46d 33' 20.5"

with an uncertainty of 2.0 arcsec (radius, 90% confidence).

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.60 (+/-0.11).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.04 (+0.18, -0.17). The
best-fitting absorption column is  3.1 (+0.7, -0.5) x 10^21 cm^-2,
consistent with the Galactic value of 2.6 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 3.7 x 10^-11 (5.7 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.1 (+0.7, -0.5) x 10^21 cm^-2
Galactic foreground: 2.6 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.04 (+0.18, -0.17)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01352124.

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