TITLE: GCN CIRCULAR NUMBER: 23373 SUBJECT: GRB 181023A: Swift-XRT refined Analysis DATE: 18/10/23 15:40:45 GMT FROM: Phil Evans at U of Leicester A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), D.N. Burrows (PSU), A. Tohuvavohu (PSU), S. J. LaPorte (PSU), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester) and F.E. Marshall report on behalf of the Swift-XRT team: We have analysed 10 ks of XRT data for GRB 181023A (Marshall et al. GCN Circ. 23366), from 214 s to 36.1 ks after the BAT trigger. The data comprise 493 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 23369). The late-time light curve (from T0+5.4 ks) can be modelled with a series of power-law decays. The initial decay index is alpha=1.56 (+0.16, -0.18). At T+12.4 ks the decay steepens to an alpha of 3.9 (+2.1, -0.4). The light curve breaks again at T+18.1 ks to a decay with alpha=-1.5 (+0.0, -0.7), before a final break at T+22.9 ks s after which the decay index is 2.9 (+0.9, -0.7). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.79 (+/-0.08). The best-fitting absorption column is 2.3 (+/-0.3) x 10^21 cm^-2, in excess of the Galactic value of 6.0 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 4.1 x 10^-11 (5.2 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.3 (+/-0.3) x 10^21 cm^-2 Galactic foreground: 6.0 x 10^20 cm^-2 Excess significance: 9.0 sigma Photon index: 1.79 (+/-0.08) If the light curve continues to decay with a power-law decay index of 2.9, the count rate at T+24 hours will be 4.5 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.8 x 10^-13 (2.4 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00868427. This circular is an official product of the Swift-XRT team.