TITLE: GCN CIRCULAR NUMBER: 18509 SUBJECT: GRB 151027B: Swift-XRT refined Analysis DATE: 15/10/28 10:57:09 GMT FROM: Phil Evans at U of Leicester V. D'Elia (ASDC), A. D'ai (INAF-IASFPA), A. Maselli (INAF-IASFPA), L.M. McCauley (PSU), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), C. Pagani (U. Leicester), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester) and T.N. Ukwatta report on behalf of the Swift-XRT team: We have analysed 12 ks of XRT data for GRB 151027B (Ukwatta et al. GCN Circ. 18499), from 191 s to 32.5 ks after the BAT trigger. The data comprise 61 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 enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 18504). The light curve can be modelled with an initial power-law decay with an index of alpha=5.7 (+/-0.6), followed by a break at T+332 s to an alpha of 0.66 (+0.05, -0.06). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.81 (+0.14, -0.13). The best-fitting absorption column is 4.1 (+2.5, -2.2) x 10^22 cm^-2, at a redshift of 4.063, in addition to the Galactic value of 9.4 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 3.8 x 10^-11 (4.8 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Galactic foreground: 9.4 x 10^20 cm^-2 Intrinsic column: 4.1 (+2.5, -2.2) x 10^22 cm^-2 at z=4.063 Photon index: 1.81 (+0.14, -0.13) If the light curve continues to decay with a power-law decay index of 0.66, the count rate at T+24 hours will be 0.026 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.9 x 10^-13 (1.2 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/00661869. This circular is an official product of the Swift-XRT team.