GCN Circular 41425

D.N. Burrows (PSU), M.A. Williams (PSU), S. Dichiara (PSU), J.P.
Osborne (U. Leicester), K.L. Page (U. Leicester), R. Brivio (INAF-OAB),
A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB) and P.A. Evans report on
behalf of the Swift-XRT team:

We have analysed 5.5 ks of XRT data for GRB 250818A, from 86 s to 83.2
ks after the   trigger. The data comprise 247 s in Windowed Timing (WT)
mode (the first 10 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+4.4 ks) can be modelled with a
power-law decay with a decay index of alpha=1.15 (+/-0.04).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.18 (+/-0.06). The
best-fitting absorption column is  1.63 (+/-0.08) x 10^22 cm^-2, in
excess of the Galactic value of 6.8 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.76 (+0.17, -0.16)
and a best-fitting absorption column of 1.69 (+0.25, -0.23) x 10^22
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 6.6 x 10^-11 (1.2 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.69 (+0.25, -0.23) x 10^22 cm^-2
Galactic foreground: 6.8 x 10^21 cm^-2
Excess significance: 7.2 sigma
Photon index:	     1.76 (+0.17, -0.16)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01343270.

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