TITLE: GCN CIRCULAR NUMBER: 19985 SUBJECT: GRB 161004A: Swift-XRT refined Analysis DATE: 16/10/04 21:47:28 GMT FROM: Phil Evans at U of Leicester P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), T.G.R. Roegiers (PSU), A.P. Beardmore (U. Leicester), S.L. Gibson (U. Leicester), A. D'Ai (INAF-IASFPA) and P.A. Evans report on behalf of the Swift-XRT team: We have analysed 6.1 ks of XRT data for GRB 161004A (Evans et al. GCN Circ. 19979), from 84 s to 18.2 ks after the BAT trigger. The data comprise 112 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. 19983). The late-time light curve (from T0+4.3 ks) can be modelled with a power-law decay with a decay index of alpha=1.3 (+0.5, -0.4). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.94 (+0.11, -0.09). The best-fitting absorption column is consistent with the Galactic value of 1.9 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.59 (+0.18, -0.16) and a best-fitting absorption column of 2.2 (+0.9, -0.4) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.6 x 10^-11 (5.5 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.2 (+0.9, -0.4) x 10^21 cm^-2 Galactic foreground: 1.9 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.59 (+0.18, -0.16) If the light curve continues to decay with a power-law decay index of 1.3, the count rate at T+24 hours will be 1.1 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.9 x 10^-14 (5.9 x 10^-14) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00715084. This circular is an official product of the Swift-XRT team.