GCN Circular 39221

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

We have analysed 6.0 ks of XRT data for GRB 250207A, from 100 s to 73.6
ks after the  BAT trigger. The data comprise 370 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 late-time light curve (from T0+4.2 ks) can be modelled with a
power-law decay with a decay index of alpha=1.22 (+0.13, -0.10).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.14 (+/-0.06). The
best-fitting absorption column is  6.7 (+/-1.2) x 10^20 cm^-2, in
excess of the Galactic value of 2.4 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.02 (+0.16, -0.15)
and a best-fitting absorption column of 5.5 (+3.5, -3.1) x 10^20 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.3 x 10^-11 (3.8 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     5.5 (+3.5, -3.1) x 10^20 cm^-2
Galactic foreground: 2.4 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.02 (+0.16, -0.15)

If the light curve continues to decay with a power-law decay index of
1.22, the count rate at T+24 hours will be 0.010 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.4 x
10^-13 (3.9 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/01287821.

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