TITLE: GCN CIRCULAR NUMBER: 20852 SUBJECT: GRB 170311A: Swift-XRT refined Analysis DATE: 17/03/12 03:29:38 GMT FROM: Phil Evans at U of Leicester A. D'Ai (INAF-IASFPA), D.N. Burrows (PSU), A. Cholden-Brown (PSU), S. J. LaPorte (PSU), K.L. Page (U. Leicester), B. Mingo (U. Leicester), A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB) and report on behalf of the Swift-XRT team: report on behalf of the Swift-XRT team: We have analysed 6.8 ks of XRT data for GRB 170311A, from 108 s to 35.6 ks after the BAT trigger. The data comprise 64 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The light curve can be modelled with a power-law decay with a decay index of alpha=1.59 (+0.16, -0.17). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.52 (+0.28, -0.26). The best-fitting absorption column is 2.2 (+1.5, -0.9) x 10^21 cm^-2, consistent with the Galactic value of 1.3 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 4.8 x 10^-11 (5.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.2 (+1.5, -0.9) x 10^21 cm^-2 Galactic foreground: 1.3 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.52 (+0.28, -0.26) If the light curve continues to decay with a power-law decay index of 1.59, the count rate at T+24 hours will be 6.1 x 10^-5 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.9 x 10^-15 (3.4 x 10^-15) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00741965. This circular is an official product of the Swift-XRT team.