TITLE: GCN CIRCULAR NUMBER: 20917 SUBJECT: GRB 170318A: Swift-XRT refined Analysis DATE: 17/03/19 04:03:37 GMT FROM: Phil Evans at U of Leicester S. J. LaPorte (PSU), J.A. Kennea (PSU), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), S.L. Gibson (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU) and report on behalf of the Swift-XRT team: report on behalf of the Swift-XRT team: We have analysed 7.0 ks of XRT data for GRB 170318A, from 98 s to 41.1 ks after the BAT trigger. The data comprise 94 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The light curve can be modelled with an initial power-law decay with an index of alpha=1.09 (+0.51, -0.29), followed by a break at T+208 s to an alpha of 2.74 (+0.76, -0.17). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 0.84 (+0.20, -0.19). The best-fitting absorption column is 6.3 (+2.7, -1.9) x 10^21 cm^-2, consistent with the Galactic value of 4.4 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 0.92 (+0.35, -0.30) and a best-fitting absorption column of 6.4 (+4.6, -1.9) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 8.3 x 10^-11 (9.3 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 6.4 (+4.6, -1.9) x 10^21 cm^-2 Galactic foreground: 4.4 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 0.92 (+0.35, -0.30) If the light curve continues to decay with a power-law decay index of 2.74, the count rate at T+24 hours will be 4.0 x 10^-7 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.4 x 10^-17 (3.8 x 10^-17) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00743065. This circular is an official product of the Swift-XRT team.