GCN Circular 10733
R. Margutti, B. Sbarufatti, A. de Ugarte Postigo (INAF-OAB) report on behalf of the Swift-XRT team: We have analysed 3.0 ks of XRT data for GRB 100508A (Margutti et al. GCN Circ. 10728), from 132 s to 7.3 ks after the BAT trigger. The data comprise 40 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The UVOT-enhanced XRT position was given by Beardmore al. in GCN Circ. 10731. The light curve can be modelled with an initial power-law decay with an index of alpha=1.49 (+0.22, -0.18), followed by a break at T+694 s to an alpha=0.39 (+0.08, -0.09). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.66 (+0.28, -0.27). The best-fitting absorption column is 1.4 (+0.9, -0.7) x 10^21 cm^-2, in excess of the Galactic value of 2.4 x 10^20 cm^-2 (Kalberla et al. 2005). Uncertainties are given at 90% c.l. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.7 x 10^-11 (5.7 x 10^-11) erg cm^-2 count^-1. If the light curve continues to decay with a power-law decay index of 0.39, the count rate at T+24 hours will be 0.068 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.2 x 10^-12 (3.9 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/00421386. This circular is an official product of the Swift-XRT team.