GCN Circular 34489

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

We have analysed 6.4 ks of XRT data for GRB 230818A, from 125 s to 40.1
ks after the  BAT trigger. The data comprise 41 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 light curve can be modelled with an initial power-law decay with an
index of alpha=2.2 (+0.5, -0.4), followed by a break at T+385 s to an
alpha of 0.76 (+/-0.06).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.96 (+0.21, -0.20). The
best-fitting absorption column is  1.8 (+/-0.7) x 10^21 cm^-2, in
excess of the Galactic value of 1.1 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.6 x 10^-11 (4.7 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.8 (+/-0.7) x 10^21 cm^-2
Galactic foreground: 1.1 x 10^21 cm^-2
Excess significance: 1.8 sigma
Photon index:	     1.96 (+0.21, -0.20)

If the light curve continues to decay with a power-law decay index of
0.76, the count rate at T+24 hours will be 0.012 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.3 x
10^-13 (5.7 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/01186032.

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