TITLE: GCN CIRCULAR NUMBER: 13529 SUBJECT: GRB 120728A: Swift-XRT refined Analysis DATE: 12/07/29 10:38:05 GMT FROM: Phil Evans at U of Leicester V. D'Elia (ASDC), A. Maselli (INAF-IASFPA), M.C. Stroh (PSU), D.N. Burrows (PSU), J.A. Kennea (PSU), P.A. Evans (U. Leicester), O.M. Littlejohns (U. Leicester), J.P. Osborne (U. Leicester), V. Mangano (INAF-IASFPA) and J.R. Cummings report on behalf of the Swift-XRT team: We have analysed 10 ks of XRT data for GRB 120728A (Cummings et al. GCN Circ. 13525), from 161 s to 24.9 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN. Circ 13528). The late-time light curve (from T0+5.1 ks) can be modelled with a power-law decay with a decay index of alpha=0.98 (+/-0.25). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.67 (+0.25, -0.10). The best-fitting absorption column is consistent with the Galactic value of 5.2 x 10^21 cm^-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.4 x 10^-11 (1.1 x 10^-10) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 0 (+2.2, -0) x 10^20 cm^-2 Galactic foreground: 5.2 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 2.67 (+0.25, -0.10) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00529021. This circular is an official product of the Swift-XRT team.