GCN Circular 12869
O. M. Littlejohns (U Leicester) reports on behalf of the Swift-XRT team: We have analysed 8.6 ks of XRT data for GRB 120118B (Littlejohns et al. GCN Circ. 12852), from 96 s to 33.9 ks after the BAT trigger. The data comprise 19 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 Osborne et al. (GCN. Circ 12857). The light curve can be modelled with a series of power-law decays. The initial decay index is alpha=3.9 (+0.5, -0.4). At T+312 s the decay flattens to an alpha of -0.25 (+0.24, -0.21) before breaking again at T+2735 s to a final decay with index alpha=0.99 (+/-0.15). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.22 (+0.20, -0.18). The best-fitting absorption column is 2.7 (+/-0.6) x 10^21 cm^-2, in excess of the Galactic value of 6.9 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 3.6 x 10^-11 (6.4 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.7 (+/-0.6) x 10^21 cm^-2 Galactic foreground: 6.9 x 10^20 cm^-2 Excess significance: 6.0 sigma Photon index: 2.22 (+0.20, -0.18) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00512003. This circular is an official product of the Swift-XRT team.