GCN Circular 40773

M.A. Williams (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester),
K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), E. Ambrosi 
(INAF-IASFPA) , M. Capalbi (INAF-OAR), M. Perri (SSDC & INAF-OAR), J.A.
Kennea (PSU) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.3 ks of XRT data for GRB 250617B, from 779 s to 31.0
ks after the   trigger. The data comprise 7 s in Windowed Timing (WT)
mode (taken while Swift was slewing), with the remainder in Photon
Counting (PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.90 (+/-0.06).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.16 (+0.19, -0.18). The
best-fitting absorption column is  1.9 (+0.6, -0.5) x 10^21 cm^-2, in
excess of the Galactic value of 9.7 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.3 x 10^-11 (4.8 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.9 (+0.6, -0.5) x 10^21 cm^-2
Galactic foreground: 9.7 x 10^20 cm^-2
Excess significance: 3.0 sigma
Photon index:	     2.16 (+0.19, -0.18)

If the light curve continues to decay with a power-law decay index of
0.90, the count rate at T+24 hours will be 0.025 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.3 x
10^-13 (1.2 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/01325580.

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