TITLE: GCN CIRCULAR NUMBER: 21836 SUBJECT: GRB 170906A: Swift-XRT refined Analysis DATE: 17/09/06 19:39:09 GMT FROM: Phil Evans at U of Leicester A. D'Ai (INAF-IASFPA), A. Tohuvavohu (PSU), S. J. LaPorte (PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB) and M.H. Siegel report on behalf of the Swift-XRT team: We have analysed 6.4 ks of XRT data for GRB 170906A (Siegel et al. GCN Circ. 21821), from 71 s to 62.4 ks after the BAT trigger. The data comprise 683 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 21824). The late-time light curve (from T0+10.2 ks) can be modelled with a power-law decay with a decay index of alpha=2.08 (+0.27, -0.25). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.64 (+/-0.03). The best-fitting absorption column is 2.60 (+0.16, -0.15) x 10^21 cm^-2, in excess of the Galactic value of 1.4 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.23 (+0.24, -0.23) and a best-fitting absorption column of 4.7 (+1.3, -1.1) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10^-11 (6.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 4.7 (+1.3, -1.1) x 10^21 cm^-2 Galactic foreground: 1.4 x 10^21 cm^-2 Excess significance: 4.9 sigma Photon index: 2.23 (+0.24, -0.23) If the light curve continues to decay with a power-law decay index of 2.08, the count rate at T+24 hours will be 2.8 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.0 x 10^-13 (1.9 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00770957. This circular is an official product of the Swift-XRT team.