GCN Circular 10877
G. Stratta (ASDC), V. D'Elia (ASDC) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift-XRT team: We have analysed 3.6 ks of XRT data for GRB 100621A (Ukwatta et al. GCN Circ. 10870), from 79 s to 7.0 ks after the BAT trigger. The data comprise 322 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Evans et al. (GCN. Circ 10872). The light curve can be modelled with a series of power-law decays. The initial decay index is alpha=3.41 (+0.11, -0.18). At T+153 s the decay steepens to an alpha of 4.34 (+0.20, -0.12) before breaking again at T+389 s to a final decay with index alpha=0.55 (+0.13, -0.07). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.73 (+0.07, -0.06). The best-fitting absorption column is 6.11 (+0.27, -0.26) x 10^21 cm^-2, in excess of the Galactic value of 2.9 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index of 2.15 (+0.17, -0.16) and a best-fitting absorption column of 6.6 (+1.0, -0.9) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.0 x 10^-11 (1.0 x 10^-10) erg cm^-2 count^-1. If the light curve continues to decay with a power-law decay index of 0.55, the count rate at T+24 hours will be 0.23 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x 10^-11 (2.4 x 10^-11) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00425151. This circular is an official product of the Swift-XRT team.