GCN Circular 10476
A. de Ugarte Postigo, R. Margutti, B. Sbarufatti (INAF-OAB) and E. Troja (NASA/GSFC/ORAU) report on behalf of the Swift-XRT team: We have analysed 2.5 ks of the Swift-XRT data for GRB 100305A (Troja et al. GCN Circ. 10470), from 145 s to 6.0 ks after the BAT trigger. The data comprise 84 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The best position of the X-ray afterglow is the UVOT enhanced XRT position given in Osborne et al. (GCN Circ. 10474). Analysis of the light curve shows an initial steep power-law decay with an index of alpha=3.5 (+/-0.4), followed by a break at T+300 s to an alpha of 0.84 (+0.11, -0.58). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.96 (+0.21,-0.20). The best-fitting absorption column is 1.7 (+/-0.5) x 10^21 cm^-2, in excess of the Galactic value of 2.1 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index of 2.14 (+0.24,-0.20) and a best-fitting absorption column of 1.7 (+0.60, -0.30) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10^-11 (5.8 x 10^-11) erg cm^-2 count^-1. If the light curve continues to decay with a power-law decay index of 0.84, the count rate at T+24 hours will be 0.015 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.8 x 10^-13 (8.7 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/00414905. This circular is an official product of the Swift-XRT team.