TITLE:   GCN CIRCULAR
NUMBER:  41170
SUBJECT: GRB 250725A: Swift-XRT refined Analysis
DATE:    25/07/25 19:07:22 GMT
FROM:    Phil Evans at U of Leicester <pae9@star.le.ac.uk>

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

We have analysed 7.4 ks of XRT data for GRB 250725A, from 828 s to 53.2
ks after the   trigger. The data comprise 64 s in Windowed Timing (WT)
mode with the remainder in Photon Counting (PC) mode. 

The light curve can be modelled with an initial power-law decay with an
index of alpha=1.14 (+/-0.06), followed by a break at T+13.4 ks to an
alpha of 2.2 (+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 1.72 (+0.09, -0.08). The
best-fitting absorption column is  2.7 (+2.3, -2.1) x 10^22 cm^-2, at a
redshift of 5.26, in addition to the Galactic value of 9.2 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 4.0 x
10^-11 (4.7 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 9.2 x 10^20 cm^-2
Intrinsic column:    2.7 (+2.3, -2.1) x 10^22 cm^-2 at z=5.26
Photon index:	     1.72 (+0.09, -0.08)

If the light curve continues to decay with a power-law decay index of
2.2, the count rate at T+24 hours will be 4.0 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.6 x
10^-13 (1.9 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/01336720.

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