TITLE:   GCN CIRCULAR
NUMBER:  40835
SUBJECT: GRB 250625A: Swift-XRT refined Analysis
DATE:    25/06/26 05:57:53 GMT
FROM:    Phil Evans at U of Leicester <pae9@star.le.ac.uk>

S. Dichiara (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester), M. Capalbi (INAF-OAR), M. Perri (SSDC & INAF-OAR), V.
D'Elia (SSDC & INAF-OAR), J.A. Kennea (PSU), D.N. Burrows (PSU) and
P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.0 ks of XRT data for GRB 250625A, from 123 s to 45.1
ks after the   trigger. The data comprise 58 s in Windowed Timing (WT)
mode with the remainder in Photon Counting (PC) mode. 

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=0.8 (+1.1, -1.2). At T+174 s  the decay
steepens to an alpha of 6.3 (+/-1.7) before breaking again at T+238 s
to a final decay with index alpha=1.19 (+0.17, -0.15).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.68 (+0.32, -0.29). The
best-fitting absorption column is  1.7 (+1.3, -1.0) x 10^21 cm^-2, in
excess of 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 4.3 x 10^-11 (5.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.7 (+1.3, -1.0) x 10^21 cm^-2
Galactic foreground: 5.5 x 10^20 cm^-2
Excess significance: 1.9 sigma
Photon index:	     1.68 (+0.32, -0.29)

If the light curve continues to decay with a power-law decay index of
1.19, the count rate at T+24 hours will be 5.0 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.2 x
10^-14 (2.6 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01327910.

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