TITLE: GCN CIRCULAR NUMBER: 11171 SUBJECT: GRB 100901A: Swift-XRT Team refined analysis DATE: 10/09/02 06:00:48 GMT FROM: Kim Page at U.of Leicester K.L. Page (U. Leicester) and S. Immler (CRESST/GSFC/UMD) report on behalf of the Swift-XRT team: We have analysed 13 ks of XRT data for GRB 100901A (Immler et al., GCN Circ. 11159), from 128 s to 33.2 ks after the BAT trigger. The data comprise 448 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 Osborne et al. (GCN. Circ 11167). The first snapshot (data collected until ~620 s after the trigger) consists of a number of superimposed flares, with the main peak around 410 s; this peak was also seen in the BAT data (Sakamoto et al., GCN Circ. 11169). The light curve then shows a decaying trend with alpha = 0.74 +/- 0.12, until around 12 ks, after which there is a small rebrightening, to a plateau level of around 0.3 count s^-1. A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.68 (+/-0.04). The best-fitting absorption column is 3.6 (+0.7, -0.6) x 10^21 cm^-2, at a redshift of 1.408 (Chornock et al., GCN Circ 11164), in addition to the Galactic value of 7.1 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index of 2.19 (+/-0.07) and a best-fitting absorption column of 3.9 (+/-0.9) x 10^21 cm^-2 (in addition to the Galactic vale). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10^-11 (5.1 x 10^-11) erg cm^-2 count^-1. Because of the rebrightening/plateau shape of the light curve at this time, we cannot sensibly predict the count rate at 24 hours. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00433065. This circular is an official product of the Swift-XRT team.