GCN Circular 12261
G. Stratta and V. D'Elia (ASDC) report on behalf of the Swift-XRT team: We have analysed 3.8 ks of XRT data for GRB 110808A (Stratta et al. GCN Circ. 12256), from 100 s to 18.0 ks after the BAT trigger. The data comprise 91 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 enhanced XRT position for this burst was given by Goad et al. (GCN. Circ 12257). The light curve can be modelled with an initial power-law decay with an index of alpha=3.79 (+0.22, -0.20), followed by a break at T+459 s to an alpha of 0.57 (+0.18, -0.23). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.07 (+0.29, -0.25). The best-fitting absorption column is 2.9 (+1.4, -1.3) x 10^21 cm^-2, at a redshift of 1.348 (de Ugarte Postigo et al. GCN Circ. 12258), in addition to the Galactic value of 1.0 x 10^20 cm^-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 2.2 x 10^-11 (3.5 x 10^-11) erg cm^-2 count^-1. A summary of the WT-mode spectrum is thus: Galactic foreground: 1.0 x 10^20 cm^-2 Intrinsic column: 2.9 (+1.4, -1.3) x 10^21 cm^-2 at z=1.348 Photon index: 3.07 (+0.29, -0.25) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00458918. This circular is an official product of the Swift-XRT team.