TITLE: GCN CIRCULAR NUMBER: 9857 SUBJECT: GRB 090831C: Swift XRT Refined Analysis DATE: 09/09/01 07:03:06 GMT FROM: Antonia Rowlinson at U.of Leicester A. Rowlinson (U. Leicester) and P. Schady (MSSL-UCL) report on behalf of the Swift-XRT team: We have analysed 8.4 ks of XRT data for GRB 090831C (Schady et al. GCN Circ. 9854), from 121 s to 21.8 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN. Circ 9855). The light curve has two flares at T+186 s and T+440 s, while the underlying decay can be modelled with a power-law decay with a decay index of alpha=0.83 (+0.17, -0.11). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.84 (+0.21, -0.15). The best-fitting absorption column is 3.254 (+1.001, -0.014) x 10^21 cm^-2, consistent with the Galactic value of 3.2 x 10^21 cm^-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.9 x 10^-11 (7.2 x 10^-11) erg cm^-2 count^-1. If the light curve continues to decay with a power-law decay index of 0.83, the count rate at T+24 hours will be 0.0029 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.4 x 10^-13 (2.1 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00361489. This circular is an official product of the Swift-XRT team.