GCN Circular 9777

R. Starling (U. Leicester) and M. Stamatikos (NASA/ORAU) report on behalf 
of the Swift-XRT team:

We have analysed the first two orbits of Swift XRT data for GRB 090812 
(trigger=359711, Stamatikos et al. GCN Circ. 9768
Not found
 
 
) spanning T0+78 s to 
T0+12.4 ks. The UVOT-enhanced XRT position was given by Goad et al. in GCN 
Circ. 9748
Not found
 
 
.

The light curve shows initial flaring, with two large flares overlaid on a 
power law of decay slope alpha1=1.5+/-0.5. The power law then breaks at 
approximately 960 s to a shallower slope of alpha=1.05+/-0.10.

A power law is an acceptable fit to a spectrum formed from the first orbit 
windowed timing (WT) mode data centred at T0+230 s, with a photon index, 
Gamma, of 1.97+/-0.03. The resulting column density at the source 
(z=2.452, de Ugarte Postigo et al. GCN Circ. 9771
Not found
 
 
) is (1.00+/-0.09)e22 
cm^-2 in addition to the Galactic column of 2.26e20 cm^-2 (Kalberla et al. 
2005). 
The 0.3-10 keV observed (unabsorbed) count to flux conversion is 1 ct = 
3.4e-11 (4.2e-11) erg/cm2/s.
A fit to the spectrum formed from the first and second orbit photon 
counting (PC) mode data centred at T0+2760 s is consistent with the WT 
spectral results.

Interestingly, the X-ray spectral slope we derive is consistent with slope 
reported for the optical data from GROND, centred at T0+7860 s (Updike et 
al. GCN Circ. 9773
Not found
 
 
.), which may imply a single spectral slope between
the X-ray and optical bands several thousand seconds after the trigger. 

While we only have two orbits of data, if we assume the light curve 
continues to decay with a power law of index alpha=1.05, the 
predicted count rate at T+24 hours will be 0.023 count s^-1, 
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.7e-13 
(9.5e-13) erg cm^-2 s^-1. 

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00359711.

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