GCN Circular 11022

C. Pagani (U. Leicester) and D. C. Morris (GWU/GSFC) report on behalf of
the Swift-XRT team:

We have analysed 6.8 ks of XRT data for GRB 100728B (Morris et al. GCN
Circ. 11009
Loading...
 
 ), from 81 s to 17.4 ks after the BAT trigger. The data
comprise 86 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 enhanced XRT position for this burst was given by Evans et al.
(GCN. Circ 11017).

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.109 (+0.029, -0.030).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.20 (+/-0.20). The
best-fitting absorption column is 1.2 (+0.4, -0.5) x 10^21 cm^-2, in
excess of the Galactic value of 6.2 x 10^20 cm^-2 (Kalberla et al.
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 3.5 x 10^-11 (5.0 x 10^-11) erg
cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of
1.109, the count rate at T+24 hours will be 5.0 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.8 x
10^-13 (2.5 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/00430172.

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