TITLE: GCN CIRCULAR NUMBER: 11003 SUBJECT: GRB 100727A: Swift XRT refined analysis DATE: 10/07/27 21:07:26 GMT FROM: Boris Sbarufatti at INAF-OAB/IASFPA B. Sbarufatti (INAF OAB/IASF PA), V. Mangano (INAF/IASF PA) and D. C. Morris (GWU/GSFC) report on behalf of the Swift-XRT team: We have analysed 9.8 ks of XRT data for GRB 100727A (Morris et al. GCN Circ. 10998), from 48 s to 39.8 ks after the BAT trigger. The data comprise 185 s in Windowed Timing (WT) mode (the first 7 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 11000). The light curve can be modeled with a doubly broken power-law decay. The initial decay index is alpha=2.6 (+/-0.1). A bright flare starts at T+190 s and peaks at T+247 s. The light curve breaks at T+450 s entering a plateau phase with alpha=0.0 (+0.06 -0.08). The second break is observed at T+6.5 ks, followed by a decay index alpha=0.64 (+0.12-0.09) A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.8 (+/-0.14). The best-fitting absorption column is 4.2 (+/-0.3) x 10^21 cm^-2, in excess of the Galactic value of 5.1 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index of 2.6 (+/-0.2) and a best-fitting absorption column of 3.1 (+/-0.5) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 9.7 x 10^-11 (2.3 x 10^-10) erg cm^-2 count^-1. If the light curve continues to decay with a power-law decay index of 0.64, the count rate at T+24 hours will be 2.8 x 10^-2 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.7 x 10-12 (6.4 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/00430094. This circular is an official product of the Swift-XRT team.