GCN Circular 37888

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

We have analysed 6.8 ks of XRT data for GRB 241025A, from 109 s to 80.1
ks after the  BAT trigger. The data comprise 321 s in Windowed Timing
(WT) mode (the first 4 s were taken while Swift was slewing) 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=8.0 (+0.0, -0.5). At T+126 s  the decay
flattens to an alpha of 3.15 (+0.20, -0.21) before breaking again at
T+221 s to a final decay with index alpha=1.071 (+0.026, -0.025).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.77 (+/-0.05). The
best-fitting absorption column is  3.6 (+/-1.1) x 10^22 cm^-2, at a
redshift of 4.2, in addition to the Galactic value of 1.2 x 10^21 cm^-2
(Willingale et al. 2013). The PC mode spectrum has a photon index of
1.68 (+/-0.12) and a best-fitting absorption column of 2.4 (+2.4, -2.1)
x 10^22 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 4.3 x 10^-11 (5.1 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 1.2 x 10^21 cm^-2
Intrinsic column:    2.4 (+2.4, -2.1) x 10^22 cm^-2 at z=4.2
Photon index:	     1.68 (+/-0.12)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01262165.

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