GCN Circular 10055
G. Stratta (ASDC) reports on behalf of the Swift-XRT team: The Swift-XRT began observing GRB 091018 (trigger=373172) on 2009-10-18 at 20:49:20.5 UT, 60.9 seconds after the BAT trigger (Stamatikos et al., GCN Circ. 7226). The data have been taken in Windowed Timing (WT) mode from T + 67 s to T + 144 s and in Photon Counting (PC) mode later on. Enhanced position of the detected, bright X-ray afterglow was given by Osborne et al. (GCN Circ. 10037) The X-ray light curve from T + 67 s to T + 53.3 ks is well fit by a broken power law model (chi^2 = 115 for 128 degrees of freedom) with a temporal break at tb = T + (500 +/- 100) s and decay indices alpha1 = -0.4 +/- 0.1 and alpha2 = -1.17 +/- 0.03 before and after the break, respectively. The 0.3-10 keV spectrum from T + 156 s to T + 7.1 ks is well fit by an absorbed power law (reduced chi2 of 1.0, with 50 degrees of freedom). The best fit photon index is 1.9 +/- 0.1, with an hydrogen column density at the burst rest frame (z=0.971, Hsiao-Wen Chen et al. GCN Circ.10038) of NHz = (1.4 +/- 0.8)e21 cm^-2 in addition to the Galactic column along the line of sight, that is NH = 2.8e20 cm^-2 (Kalberla et al. 2005). Errors are given at 90% confidence level. The observed (unabsorbed) flux in the 0.3-10 keV band is 1.5(1.8)e-10 erg cm^-2 s^-1. This circular is an official product of the Swift-XRT team.