TITLE: GCN CIRCULAR NUMBER: 34287 SUBJECT: GRB 230728A: Swift-XRT refined Analysis DATE: 23/07/28 12:26:15 GMT FROM: Phil Evans at U of Leicester J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), B. Sbarufatti (INAF-OAB), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAR), J. D. Gropp (PSU), S. Dichiara (PSU), J.A. Kennea (PSU), A.P. Beardmore (U. Leicester) and P.A. Evans report on behalf of the Swift-XRT team: We have analysed 6.7 ks of XRT data for GRB 230728A, from 86 s to 28.9 ks after the BAT trigger. The data comprise 94 s in Windowed Timing (WT) mode (the first 3 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The late-time light curve (from T0+5.6 ks) can be modelled with a power-law decay with a decay index of alpha=1.19 (+0.20, -0.22). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.97 (+0.24, -0.23). The best-fitting absorption column is 4.4 (+1.4, -1.2) x 10^21 cm^-2, in excess of the Galactic value of 6.3 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.04 (+0.19, -0.18) and a best-fitting absorption column of 7.0 (+1.5, -1.3) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.3 x 10^-11 (7.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 7.0 (+1.5, -1.3) x 10^21 cm^-2 Galactic foreground: 6.3 x 10^20 cm^-2 Excess significance: 8.1 sigma Photon index: 2.04 (+0.19, -0.18) If the light curve continues to decay with a power-law decay index of 1.19, the count rate at T+24 hours will be 6.2 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.6 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/01181187. This circular is an official product of the Swift-XRT team.