GCN Circular 9750

R. Margutti (Univ. Bicocca & INAF-OAB), J. Mao (INAF-OAB),
A. de Ugarte Postigo (INAF-OAB)  and J. K. Cannizzo (NASA/UMBC)
report on behalf of the Swift-XRT team:

We have analysed the first orbit of Swift-XRT data obtained for
GRB 090807 (Cannizzo et al. GCN Circ. 9746
Not found
 
 
), covering 149 s of
Windowed Timing (WT) and 1.5 ks of Photon Counting (PC) mode data,
respectively, between 154 s and 1.9 ks after the trigger.
The UVOT-enhanced XRT position was given by Beardmore et al. in
GCN Circ. 9748
Not found
 
 
.

The light curve shows a large flare peaking around 185 s which was
also detected in the 15-150 keV energy range by Swift/BAT (Ukwatta et
al. GCN Circ. 9749
Not found
 
 
). The underlying light curve decays
with a power-law index of 5.0 +/- 0.5. Around T+580 s the decay flattens,
to a slope of 0.87 +/- 0.42.

The spectrum extracted from WT data can be modelled with an absorbed simple
power-law, with Gamma = 2.2 +/- 0.1 and NH = (3.0+/-0.2)x10^21 cm^-2 in
excess of the Galactic column in this direction which is 8.6x10^20 cm^-2
(Kalberla et al. 2005).
A spectrum extracted from PC data in the time interval T+310 s - T+1840 s
has a best-fitting photon index of 2.6 +/- 0.4 and a best-fitting 
absorption
column of NH = (3.3+/-0.1)x10^21 cm^-2 in excess of the Galactic value.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 3.1 x 10^-11 (8.4 x 10^-11) erg cm^-2 
count^-1.

If the light curve continues to decay with a power-law decay index
of 0.9, the count rate at T+24 hours will be 0.002 count s^-1, 
corresponding
to an observed (unabsorbed) 0.3-10 keV flux of 6.2 x 10^-14 (1.7 x 10^-13)
erg cm^-2 s^-1. However, the light-curve is possible to break before 
this time,
making this prediction unreliable.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00359378.

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