GCN Circular 10145

A. P. Beardmore (U. Leicester) and S. R. Oates (UCL-MSSL) report on
behalf of the Swift-XRT team:

We have analysed 7.9 ks of Swift-XRT data for GRB 091109 (Oates et al. 
GCN Circ. 10138
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 ), from 154 s to 18.1 ks after the BAT trigger. The data
span four orbits, comprising 37 s in Windowed Timing (WT) mode with
the remainder in Photon Counting (PC) mode. The enhanced XRT position
for this burst was given by Beardmore et al. (GCN. Circ 10140).

The light curve can be modelled by a broken power-law with an initial
decay slope of 2.9 +0.5 -0.4, breaking at T+358 s to a shallower decay
of 0.86 +0.07 -0.08.

A spectrum formed from the PC mode data (from 175 s to 18.1 ks after
the trigger) can be fitted with an absorbed power-law, giving a photon
index of 2.10 +/- 0.19. The best-fitting absorption column is (9.9
+2.1 -4.4) x 10^20 cm^-2, in excess of the Galactic value of 3.0 x
10^20 cm^-2 (Kalberla et al. 2005) in the direction of the burst. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 3.6 x 10^-11 (4.8 x 10^-11) erg cm^-2
count^-1.

If the light curve continues to decay with a power-law decay index of
0.86, the count rate at T+24 hours will be 4.7 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x
10^-13 (2.3 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/00375246.

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