TITLE: GCN CIRCULAR NUMBER: 11244 SUBJECT: GRB 100906A: Swift-XRT refined analysis DATE: 10/09/07 03:58:18 GMT FROM: Andy Beardmore at U Leicester A. P. Beardmore (U. Leicester) and C. B. Markwardt (NASA/GSFC) report on behalf of the Swift-XRT team: We have analysed 16.6 ks of XRT data for GRB 100906A (Markwardt et al. GCN Circ. 11227), from 70 s to 40.2 ks after the BAT trigger. The data comprise 228 s in Windowed Timing (WT) mode (the first 9 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 Goad et al. (GCN. Circ 11243). The X-ray light curve initially shows a large flare, peaking at a count rate of ~2000 count s^-1 at T+120s, contemporary with the late time BAT activity (Barthelmy et al., GCN Circ. 11233). It then declines rapidly before breaking to a shallower decay slope of alpha = 0.74 +/- 0.04 after T+270s. This is followed by a further break at T+10854 s to an slope of 1.96 +0.17 -0.13. A spectrum formed from the PC mode data can be fit by an absorbed power-law with a photon index of 2.15 +/-0.08. The best-fitting absorption column is (8.1 +/- 2.5) x 10^21 cm^-2, at a redshift of 1.727 (Tanvir et al. GCN Circ. 11230), in addition to the Galactic value of 2.2 x 10^21 cm^-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.1 x 10^-11 (7.1 x 10^-11) erg cm^-2 count^-1. We do not report a WT time averaged spectral fit here as this would be affected by the spectral evolution during the flaring activity, evident in the hardness ratio. If the light curve continues to decay at the same rate, the count rate at T+24 hours will be 5.9 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.4 x 10^-13 (4.2 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00433509. This circular is an official product of the Swift-XRT team.