TITLE: GCN CIRCULAR NUMBER: 11223 SUBJECT: GRB 100905A: Swift-XRT refined analysis. DATE: 10/09/06 08:08:17 GMT FROM: Phil Evans at U of Leicester P.A. Evans (U. Leicester) and F. E. Marshall (NASA/GSFC) report on behalf of the Swift-XRT team: We have analysed 7.1 ks of XRT data for GRB 100905A (Marshall et al. GCN Circ. 11214), from 78 s to 13.6 ks after the BAT trigger. The data comprise 71 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN. Circ 11219). The light curve shows some initial flaring activity, with the flares peaking at around 290, 320 and 400 s after the trigger. The flares peak an order of magnitude brighter than the underlying emission, which can be modelled with a single power-law, with a decay index of alpha=0.86 (+/-0.06). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.0 (+/-0.3). The best-fitting absorption column is 1.9 (+0.6, -0.5) x 10^21 cm^-2, in excess of the Galactic value of 5.4 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index of 1.97 (+0.12, -0.18) and a best-fitting absorption column of 8.4 (+4.1, -2.8) x 10^20 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.9 x 10^-11 (4.8 x 10^-11) erg cm^-2 count^-1. If the light curve continues to decay with a power-law decay index of 0.86, the count rate at T+24 hours will be 2.7 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x 10^-13 (1.3 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/00433442. This circular is an official product of the Swift-XRT team.