GCN Circular 34287

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), B. Sbarufatti
(INAF-OAB), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAR), J. D. Gropp
(PSU), S. Dichiara (PSU), J.A. Kennea (PSU), A.P. Beardmore (U.
Leicester) and P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.7 ks of XRT data for GRB 230728A, from 86 s to 28.9
ks after the  BAT trigger. The data comprise 94 s in Windowed Timing
(WT) mode (the first 3 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+5.6 ks) can be modelled with a
power-law decay with a decay index of alpha=1.19 (+0.20, -0.22).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.97 (+0.24, -0.23). The
best-fitting absorption column is  4.4 (+1.4, -1.2) x 10^21 cm^-2, in
excess of the Galactic value of 6.3 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.04 (+0.19, -0.18)
and a best-fitting absorption column of 7.0 (+1.5, -1.3) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.3 x 10^-11 (7.6 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7.0 (+1.5, -1.3) x 10^21 cm^-2
Galactic foreground: 6.3 x 10^20 cm^-2
Excess significance: 8.1 sigma
Photon index:	     2.04 (+0.19, -0.18)

If the light curve continues to decay with a power-law decay index of
1.19, the count rate at T+24 hours will be 6.2 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.6 x
10^-13 (4.7 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/01181187.

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