GCN Circular 8244
R.L.C. Starling (U. Leicester) and H. Ziaeepour (MSSL-UCL) report on behalf of the Swift-XRT team: We have analysed the first two orbits of Swift XRT data for GRB 080916A (Ziaeepour et al., GCN Circ. 8237) from T0+70 s to T0+7440 s with total exposure time 3.4 ks. The enhanced X-ray position is given in Evans et al., GCN Circ. 8241. The light curve can be represented by a 5-segment broken power law with an initial decay alpha1=1.3+0.4/-0.6 breaking at Tbk1=102+4/-6 s to a steep decay of alpha2=3.5+/-0.3 until Tbk2=190+25/-13 s. The light curve then flattens to alpha3=1.07+0.06/-0.3 and breaks at Tbk3=690+150/-140 s to a further steep decay of alpha4=2.9+2/-0.8. From Tbk4=1270+240/-220 s the decay becomes shallow again with alpha5=0.3+/-0.2. The spectrum from 3.2 ks of Photon Counting mode data can be fit with an absorbed power law with photon index Gamma=2.1+/-0.2 and intrinsic absorption (in excess of the Galactic value of 1.8e20 cm-2) of nH=(2.6+0.7/-0.6)e21 cm-2. This is consistent with the spectrum formed from the initial 170 s of Windowed Timing mode data. The observed (unabsorbed) count rate to flux conversion is 1 count/s = 4.0e-11 (6.8e-11) erg/cm2/s. The predicted count rate at T+24hr would be approximately 0.1 count/s if the decay continues on the current plateau-like phase. This circular is an official product of the Swift-XRT team.