GCN Circular 11498
K.L. Page (U. Leicester) and J. L. Racusin (NASA/GSFC) report on behalf of the Swift-XRT team: We have analysed 13 ks of XRT data for possible GRB 101225A (Racusin et al., GCN Circ. 11493), from 1.4 ks to 35.6 ks after the BAT trigger. The data comprise 2.7 ks 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 11497). The light curve shows substantial structure, but the underlying continuum can be modelled with a broken power-law decay. The initial decay index is alpha= 1.108 (+/-0.011). At T+21682 s the decay steepens to an alpha of 5.93 (+/-0.30). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.71 (+/-0.04). The best-fitting absorption column is 1.78 (+/-0.11) x 10^21 cm^-2, in excess of the Galactic value of 7.9 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index of 1.96 (+/-0.06) and a best-fitting absorption column of 1.58 (+0.16, -0.15) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.1 x 10^-11 (5.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.58 (+0.16, -0.15) x 10^21 cm^-2 Galactic foreground: 7.9 x 10^20 cm^-2 Excess significance: 17.1 sigma Photon index: 1.96 (+/-0.06) If the light curve continues to decay with a power-law decay index of 5.59, the count rate at T+24 hours will be 0.0007 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.7 x 10^-14 (3.7 x 10^-14) erg cm^-2 s^-1. We note that this is the brightest afterglow that the XRT has ever observed at a few thousand seconds after the trigger. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00441015. This circular is an official product of the Swift-XRT team.