GCN Circular 41280

A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), M. Ferro
(INAF-OAB), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), J.A. Kennea
(PSU), D.N. Burrows (PSU), M.A. Williams (PSU) and P.A. Evans report on
behalf of the Swift-XRT team:

We have analysed 6.3 ks of XRT data for GRB 250807B, from 133 s to 50.6
ks after the   trigger. The data comprise 188 s in Windowed Timing (WT)
mode (the first 9 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+3.8 ks) can be modelled with a
power-law decay with a decay index of alpha=1.16 (+0.14, -0.13).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.96 (+/-0.07). The
best-fitting absorption column is  4.2 (+1.5, -1.4) x 10^20 cm^-2, in
excess of the Galactic value of 1.9 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.97 (+0.21, -0.19)
and a best-fitting absorption column of 5.8 (+4.7, -4.0) x 10^20 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.4 x 10^-11 (3.8 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     5.8 (+4.7, -4.0) x 10^20 cm^-2
Galactic foreground: 1.9 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.97 (+0.21, -0.19)

If the light curve continues to decay with a power-law decay index of
1.16, the count rate at T+24 hours will be 6.4 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.1 x
10^-13 (2.4 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/01340561.

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