TITLE: GCN CIRCULAR NUMBER: 22183 SUBJECT: GRB 171205A: Swift-XRT refined Analysis DATE: 17/12/05 16:08:33 GMT FROM: Phil Evans at U of Leicester J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), P.A. Evans (U. Leicester), S.L. Gibson (U. Leicester), J.P. Osborne (U. Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB) and V. D'Elia report on behalf of the Swift-XRT team: We have analysed 6.6 ks of XRT data for GRB 171205A (D'Elia et al. GCN Circ. 22177), from 134 s to 22.5 ks after the BAT trigger. The data comprise 238 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. 22179). The light curve can be modelled with a series of power-law decays. The initial decay index is alpha=1.84 (+0.10, -0.18). At T+289 s the decay steepens to an alpha of 2.43 (+0.09, -0.08) before breaking again at T+5714 s to a final decay with index alpha=-0.16 (+0.28, -0.69). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.717 (+0.035, -0.024). The best-fitting absorption column is consistent with the Galactic value of 5.9 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.7 x 10^-11 (4.1 x 10^-11) erg cm^-2 count^-1. A summary of the WT-mode spectrum is thus: Galactic foreground: 5.9 x 10^20 cm^-2 Intrinsic column: 5.89 (+/-0.22) x 10^20 cm^-2 at z=0.0368 Photon index: 1.717 (+0.035, -0.024) If the light curve continues to decay with a power-law decay index of -0.16, the count rate at T+24 hours will be 0.025 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.1 x 10^-13 (1.0 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/00794972. This circular is an official product of the Swift-XRT team.