TITLE: GCN CIRCULAR NUMBER: 11144 SUBJECT: GRB 100823A: Swift XRT refined analysis DATE: 10/08/24 13:59:54 GMT FROM: Vanessa Mangano at INAF-IASFPA V. Mangano (INAF IASF-PA) and B. Sbarufatti (INAF OAB/INAF IASF-PA) report on behalf of the Swift-XRT team: We have analyzed 8.7 ks of XRT data for GRB 100823A (Mangano et al. GCN Circ. 11135), from 59 s to 22 ks after the BAT trigger. The data comprise 68 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 light curve can be modeled with a broken power-law decay with an initial decay index of alpha1=2.9 (+/-0.2) , a break at T+163 (+/-11) s and a final decay index of alpha2=1.19 (+/-0.03). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.6 (+0.8, -0.6). The best-fitting absorption column is 2.8 (+1.2, -0.9) x 10^21 cm^-2, in excess of the Galactic value of 3.7 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum (extracted from T+137.5 s to T+22.3 ks, 8.65 ks exposure) has a photon index of 2.2 (+0.3, -0.2) and a best-fitting absorption column of 2.1 (+0.6, -0.5) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.0 x 10^-11 (6.6 x 10^-11) erg cm^-2 count^-1. If the light curve continues to decay with a power-law decay index of 0.87, the count rate at T+48 hours will be 9.0 x 10^-4 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.6 x 10-14 (5.9 x 10^-14) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00432420. This circular is an official product of the Swift-XRT team.