TITLE: GCN CIRCULAR NUMBER: 11194 SUBJECT: GRB 100902A: Swift XRT refined analysis DATE: 10/09/03 13:12:51 GMT FROM: Rhaana Starling at U of Leicester R.L.C. Starling (U. Leicester) and T. Sakamoto (NASA/UMBC) report on behalf of the Swift-XRT team: We have analysed 7.9 ks of XRT data for GRB 100902A (Sakamoto et al. GCN Circ. 11181), from 306 s to 18.2 ks after the BAT trigger. The data comprise 306 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN. Circ 11188). The light curve can be modelled with an initial power-law decay with an index of alpha=2.4 (+/-0.1), followed by a break at T+1730 (+/-210) s to an alpha of 0.8 (+/-0.1). Overlaid on this broken power law decay are two large flares, which peak at 400 and 420 s with count rates of 1390 (+/-140) and 1440 (+/-150) count s^-1 respectively. A spectrum formed from all the PC mode data to date can be fitted with an absorbed power-law with a photon spectral index of 2.5 (+/-0.2). The best-fitting absorption column is 3.39 (+/-0.6) x 10^21 cm^-2, in excess of the Galactic value of 1.1 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 3.8 x 10^-11 (9.7 x 10^-11) erg cm^-2 count^-1. We do not report a WT time averaged spectral fit here as this would be affected by the spectral evolution during the flaring activity, evident in the hardness ratio. If the light curve continues to decay with a power-law decay index of 0.8, the count rate at T+24 hours will be 0.013 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.7 x 10^-13 (1.4 x 10^-12) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00433160. This circular is an official product of the Swift-XRT team.