TITLE: GCN CIRCULAR NUMBER: 18544 SUBJECT: GRB 151031A: Swift-XRT refined Analysis DATE: 15/10/31 18:07:05 GMT FROM: Phil Evans at U of Leicester A. Maselli (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), T.G.R. Roegiers (PSU), P.A. Evans (U. Leicester), S.L. Gibson (U. Leicester), J.P. Osborne (U. Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB) and E. Sonbas report on behalf of the Swift-XRT team: We have analysed 7.5 ks of XRT data for GRB 151031A (Sonbas et al. GCN Circ. 18538), from 443 s to 25.5 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 18541). The light curve can be modelled with a power-law decay with a decay index of alpha=0.56 (+/-0.07). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.13 (+0.22, -0.21). The best-fitting absorption column is 9.4 (+3.8, -3.3) x 10^21 cm^-2, at a redshift of 1.167, in addition to the Galactic value of 2.6 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 3.2 x 10^-11 (4.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Galactic foreground: 2.6 x 10^20 cm^-2 Intrinsic column: 9.4 (+3.8, -3.3) x 10^21 cm^-2 at z=1.167 Photon index: 2.13 (+0.22, -0.21) If the light curve continues to decay with a power-law decay index of 0.56, the count rate at T+24 hours will be 0.010 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.3 x 10^-13 (4.7 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/00662330. This circular is an official product of the Swift-XRT team.