GCN Circular 10674
M. C. Stroh, D. Grupe and E. A. Hoversten (PSU) report on behalf of the Swift-XRT team: We have analysed 641 s of XRT data for GRB 100424A (Hoversten et al. GCN Circ. 10667), from 126 s to 769 s after the BAT trigger. The data comprise 509 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The light curve can be modelled with a series of power-law decays. The initial decay index is alpha=0.63 (+/-0.14). At T+288 s the decay steepens to an alpha of 1.41 (+0.27, -0.19) before breaking again at T+526 s to a final decay with index alpha=2.8 (+0.9, -0.6). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.26 (+/-0.06). The best-fitting absorption column is 1.62 (+0.22, -0.21) x 10^21 cm^-2, in excess of the Galactic value of 2.3 x 10^20 cm^-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.8 x 10^-11 (6.5 x 10^-11) erg cm^-2 count^-1. If the light curve continues to decay with a power-law decay index of 2.8, the count rate at T+24 hours will be 5.1 x 10^-6 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.0 x 10^-16 (3.3 x 10^-16) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00420367. This circular is an official product of the Swift-XRT team.