GCN Circular 41531

S. Dichiara (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester), S. Campana (INAF-OAB), A. Melandri (INAF-OAR), T. Sbarrato
(INAF-OAB), J.A. Kennea (PSU), D.N. Burrows (PSU) and P.A. Evans report
on behalf of the Swift-XRT team:

We have analysed 4.8 ks of XRT data for GRB 250823A, from 111 s to 44.7
ks after the   trigger. The data comprise 313 s in Windowed Timing (WT)
mode (the first 7 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
light curve initially rises, with an index alpha=-1.5 (+0.0, -0.6). At
T+120 s it breaks to an alpha of 3.33 (+/-0.09). The light curve breaks
again at T+349 s to a decay with alpha=2.31 (+0.23, -0.21),  before a
final break at T+1667 s s after which the decay index is 0.78 (+0.13,
-0.12).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.91 (+/-0.04). The
best-fitting absorption column is  1.99 (+0.16, -0.15) x 10^21 cm^-2,
in excess of the Galactic value of 1.1 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.92 (+0.28, -0.26)
and a best-fitting absorption column of 1.9 (+1.0, -0.8) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.8 x 10^-11 (5.0 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.9 (+1.0, -0.8) x 10^21 cm^-2
Galactic foreground: 1.1 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.92 (+0.28, -0.26)

If the light curve continues to decay with a power-law decay index of
0.78, the count rate at T+24 hours will be 9.6 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.6 x
10^-13 (4.8 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/01344586.

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