GCN Circular 37294

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

We have analysed 8.6 ks of XRT data for GRB 240825A, from 71 s to 56.6
ks after the  BAT trigger. The data comprise 1.2 ks in Windowed Timing
(WT) mode (the first 10 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=3.13 (+0.24, -0.21). At T+119 s  the decay
flattens to an alpha of 0.83 (+/-0.06). The light curve breaks again at
T+406 s to a decay with alpha=1.121 (+0.019, -0.028),  and  again at
T+1288 s s to alpha=1.52 (+6.48, -0.05),  before a final break at
T+6251 s s after which the decay index is 1.25 (+0.06, -0.08).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.661 (+/-0.017). The
best-fitting absorption column is  1.09 (+/-0.04) x 10^22 cm^-2, at a
redshift of 0.659, in addition to the Galactic value of 5.3 x 10^20
cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index
of 1.76 (+/-0.09) and a best-fitting absorption column of 9.1 (+1.6,
-1.4) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV
flux conversion factor deduced from this spectrum  is 4.4 x 10^-11 (5.9
x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Galactic foreground: 5.3 x 10^20 cm^-2
Intrinsic column:    9.1 (+1.6, -1.4) x 10^21 cm^-2 at z=0.659
Photon index:	     1.76 (+/-0.09)

If the light curve continues to decay with a power-law decay index of
1.25, the count rate at T+24 hours will be 0.093 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.1 x
10^-12 (5.5 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01250617.

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