GCN Circular 12766
P. D'Avanzo (INAF-OAB) and T. N. Ukwatta (MSU) report on behalf of the Swift-XRT team: We have analysed 6.4 ks of XRT data for GRB 111228A (Ukwatta et al. GCN Circ. 12737), from 135 s to 29.3 ks after the BAT trigger. The data comprise 331 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 12747). The light curve can be modelled with a series of power-law decays. The initial decay index is alpha=5.41 (+0.09, -0.07). At T+418 s the decay flattens to an alpha of 0.26 (+0.05, -0.06) before breaking again at T+6950 s to a final decay with index alpha=1.27 (+0.19, -0.15). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 4.16 (+0.12, -0.11). The best-fitting absorption column is 2.92 (+0.18, -0.17) x 10^21 cm^-2, in excess of the Galactic value of 3.0 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index of 2.19 (+0.11, -0.10) and a best-fitting absorption column of 1.52 (+0.27, -0.26) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.4 x 10^-11 (5.2 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.52 (+0.27, -0.26) x 10^21 cm^-2 Galactic foreground: 3.0 x 10^20 cm^-2 Excess significance: 7.9 sigma Photon index: 2.19 (+0.11, -0.10) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00510649. This circular is an official product of the Swift-XRT team.