TITLE: GCN CIRCULAR NUMBER: 23156 SUBJECT: GRB 180818B: Swift-XRT refined Analysis DATE: 18/08/19 05:11:20 GMT FROM: Phil Evans at U of Leicester A. D'Ai (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), A. Tohuvavohu (PSU), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), S.L. Gibson (U. Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB) and F.E. Marshall report on behalf of the Swift-XRT team: We have analysed 8.1 ks of XRT data for GRB 180818B (Marshall et al. GCN Circ. 23149), from 210 s to 52.8 ks after the BAT trigger. The data comprise 205 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. 23151). The late-time light curve (from T0+4.5 ks) can be modelled with a power-law decay with a decay index of alpha=0.86 (+/-0.07). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.47 (+0.12, -0.11). The best-fitting absorption column is 2.89 (+0.25, -0.23) x 10^21 cm^-2, in excess of the Galactic value of 1.3 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.77 (+0.07, -0.06) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10^-11 (4.5 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.27 (+0.11, -0.00) x 10^21 cm^-2 Galactic foreground: 1.3 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.77 (+0.07, -0.06) If the light curve continues to decay with a power-law decay index of 0.86, the count rate at T+24 hours will be 0.030 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x 10^-12 (1.3 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/00853882. This circular is an official product of the Swift-XRT team.