TITLE: GCN CIRCULAR NUMBER: 9777 SUBJECT: GRB 090812: Swift XRT refined analysis DATE: 09/08/12 15:19:27 GMT FROM: Rhaana Starling at U of Leicester 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) spanning T0+78 s to T0+12.4 ks. The UVOT-enhanced XRT position was given by Goad et al. in GCN Circ. 9748. 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) 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.), 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.