GCN Circular 9894
M. Perri, V. D'Elia, G. Stratta (ASDC) report on behalf of the Swift-XRT team: We have analysed the first 12 ks of XRT data for GRB 090904A (Perri et al., GCN Circ. 9879), from 134 s to 29.6 ks after the BAT trigger. The enhanced XRT position for this burst was given by Evans (GCN. Circ. 9891). During the first Swift orbit, the light-curve shows a steep decay with a large superimposed flare peaking at about T+300 s. Starting from T+1000 s, the curve is well described by a broken power-law model with temporal decay indices alpha1=0.2+/-0.1, alpha2=0.9+/-0.3 and a temporal break at around T+12 ks. A spectrum extracted from the PC mode data from T+460 s to T+29.6 ks can be fitted with an absorbed power-law model with a photon spectral index of 2.46 (+0.16,-0.26) and an absorption column density of 1.03 (+0.66,-0.64)e21 cm-2 in excess of the Galactic value of 9.3e20 cm-2 (Kalberla et al. 2005). The counts-to-observed (-unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.3e-11 (6.2e-11) erg cm-2 count-1. Errors are given at the 90% confidence level. If the light curve continues to decay with a power-law index of 0.9, the count rate at T+24 hours will be 6e-3 count s-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.0e-13 (3.7e-13) erg cm-2 s-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00361830. This circular is an official product of the Swift-XRT team.