TITLE:   GCN CIRCULAR
NUMBER:  10155
SUBJECT: GRB 091109B: Swift-XRT Team refined analysis 
DATE:    09/11/10 07:33:26 GMT
FROM:    Kim Page at U.of Leicester  <kpa@star.le.ac.uk>

K.L. Page (U. Leicester) and S.R. Oates (UCL-MSSL) report on behalf of 
the Swift-XRT team:

We have analysed 9.7 ks of XRT data for GRB 091109B (Oates et al. GCN 
Circ. 10148), from 86 s to 22.9 ks after the BAT trigger. The data are 
entirely in Photon Counting (PC) mode. The enhanced XRT position for this 
burst was given by Evans et al. (GCN. Circ 10151).

The light curve can be modelled with a power-law decay with a decay index 
of alpha=0.62 (+0.11, -0.10). Although there is some evidence that there 
may be a steep, shallow, normal trend in the light-curve, as is often 
seen, there are not enough data points to warrant such additional 
modelling at this time.

A spectrum formed from the PC mode data can be fitted with an absorbed 
power-law with a photon spectral index of 2.0 (+0.5, -0.4). The 
best-fitting absorption column is 1.2 (+1.4, -0.1) x 10^21 cm^-2, in 
excess of the Galactic value of 9.2 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 3.8 x 10^-11 (5.3 x 10^-11) erg cm^-2 
count^-1.

If the light curve continues to decay with a power-law decay index of 
0.62, the count rate at T+24 hours will be 2.4 x 10^-3 count s^-1, 
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.1 x 10^-14 
(1.2 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/00375409.

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