GCN Circular 21845
B. Sbarufatti (PSU) reports on behalf of the Swift-XRT team: We have analysed 831 s of XRT data for GRB 170906B (Sbarufatti et al. GCN Circ. 21822), from 63 s to 924 s after the BAT trigger. The data comprise 9 s in Windowed Timing (WT) mode (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. 21823). The light curve can be modelled with a power-law decay with a decay index of alpha=0.63 (+0.13, -0.14). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.80 (+0.30, -0.27). The best-fitting absorption column is 7.5 (+2.8, -2.2) x 10^21 cm^-2, in excess of the Galactic value of 1.7 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.0 x 10^-11 (7.8 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 7.5 (+2.8, -2.2) x 10^21 cm^-2 Galactic foreground: 1.7 x 10^21 cm^-2 Excess significance: 4.3 sigma Photon index: 1.80 (+0.30, -0.27) If the light curve continues to decay with a power-law decay index of 0.63, the count rate at T+24 hours will be 0.033 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x 10^-12 (2.6 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/00770958. This circular is an official product of the Swift-XRT team.