GCN Circular 9759

J. Mao, R. Margutti, A. de Ugarte Postigo (INAF-OAB) and C. B. Markwardt 
(CRESST/GSFC/UMD) report on behalf of the Swift-XRT team:

We have analysed 8.6 ks of XRT data for GRB 090809 (Markwardt et al. GCN 
Circ. 9754
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 ), from 107 s to 23.3 ks after the BAT trigger. The data 
comprise 59 s in Windowed Timing (WT) mode with the remainder in Photon 
Counting (PC) mode. The enhanced XRT position for this burst was given 
by Evans et al. (GCN. Circ 9757).

The light curve shows a first flare evidence peaking around 189s, then, 
it shows a second flare peaking around 4.6ks, these flares make an order 
of magnitude increase in count-rate, after that, the light curve decays 
continuously.  In general, the underlying light curve can be modelled 
with a power-law decay with a decay index of alpha=1.02+/-0.02.

A spectrum extracted from the WT mode data can be fitted with an 
absorbed power-law with a photon spectral index of 1.75 (+/-0.09). The 
best-fitting absorption column is 2.4 (+/-0.3) x 1021 cm^-2, in excess 
of the Galactic value of 6.1 x 1020 cm^-2 (Kalberla et al. 2005). The PC 
mode spectrum has a photon index of 1.84 (+0.14, -0.13) and a 
best-fitting absorption column of 1.18 (+0.19, -0.18) x 1021 cm^-2. The 
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor 
deduced from this spectrum is 1.08 x 10^-11 (1.24
x 10^-11) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of 
1.0, the count rate at T+24 hours will be 0.007 count s^-1, 
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.9 x 
10^-13 (3.4 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/00359530.

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