TITLE: GCN CIRCULAR NUMBER: 20918 SUBJECT: GRB 170318B: Swift-XRT refined Analysis DATE: 17/03/19 05:09:13 GMT FROM: Phil Evans at U of Leicester J.A. Kennea (PSU), P.A. Evans (U. Leicester), S.L. Gibson (U. Leicester), J.P. Osborne (U. Leicester), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU) and report on behalf of the Swift-XRT team: report on behalf of the Swift-XRT team: We have analysed 7.2 ks of XRT data for GRB 170318B, from 64 s to 41.1 ks after the BAT trigger. The data comprise 95 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 rise, with a power-law index of alpha=-0.5 (+0.4, -0.5), followed by a break at T+134 s to an alpha of 2.96 (+0.16, -0.15). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.6 (+/-0.4). The best-fitting absorption column is 6.4 (+1.7, -1.4) x 10^22 cm^-2, in excess of the Galactic value of 1.4 x 10^22 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.3 (+/-0.6) and a best-fitting absorption column of 6.2 (+2.1, -1.7) x 10^22 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 8.0 x 10^-11 (3.5 x 10^-10) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 6.2 (+2.1, -1.7) x 10^22 cm^-2 Galactic foreground: 1.4 x 10^22 cm^-2 Excess significance: 4.6 sigma Photon index: 2.3 (+/-0.6) If the light curve continues to decay with a power-law decay index of 2.96, the count rate at T+24 hours will be 4.5 x 10^-8 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.6 x 10^-18 (1.6 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/00743086. This circular is an official product of the Swift-XRT team.