TITLE: GCN CIRCULAR NUMBER: 26609 SUBJECT: GRB 191228A: Swift-XRT refined Analysis DATE: 19/12/28 19:22:36 GMT FROM: Phil Evans at U of Leicester J. D. Gropp (PSU), A. Tohuvavohu (U. Toronto), J.A. Kennea (PSU), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-IASFPA) and M.G. Bernardini report on behalf of the Swift-XRT team: We have analysed 7.7 ks of XRT data for GRB 191228A (Bernardini et al. GCN Circ. 26601), from 127 s to 48.0 ks after the BAT trigger. The data comprise 216 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN Circ. 26606). The late-time light curve (from T0+5.4 ks) can be modelled with a power-law decay with a decay index of alpha=1.58 (+0.13, -0.12). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.96 (+/-0.05). The best-fitting absorption column is 3.09 (+0.24, -0.23) x 10^21 cm^-2, in excess of the Galactic value of 3.8 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.14 (+0.16, -0.15) and a best-fitting absorption column of 4.0 (+/-0.7) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (6.1 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 4.0 (+/-0.7) x 10^21 cm^-2 Galactic foreground: 3.8 x 10^20 cm^-2 Excess significance: 8.7 sigma Photon index: 2.14 (+0.16, -0.15) If the light curve continues to decay with a power-law decay index of 1.58, the count rate at T+24 hours will be 8.1 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.9 x 10^-13 (5.0 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/00946476. This circular is an official product of the Swift-XRT team.