TITLE: GCN CIRCULAR NUMBER: 9893 SUBJECT: GRB 090904B: Swift XRT Refined Analysis DATE: 09/09/04 16:49:07 GMT FROM: Valerio D'Elia at INAF-OAR V. D'Elia, G. Stratta, M. Perri (ASDC) report on behalf of the Swift-XRT team: We have analysed 6.2 ks of XRT data for GRB 090904B (D'Elia et al. GCN Circ. 9881), from 159 s to 37.1 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN. Circ 9987). The light curve, after an initial rise, peaks at T+260 s, while the following decay can be modelled with a power-law with a decay index of alpha=1.20+-0.05 (90% confidence level). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.67 (+0.26, -0.24, 90% confidence level). The best-fitting absorption column is 6.3 (+2.5, -1.1) x 1021 cm-2, in excess with respect to the Galactic value of 3.3 x 1021 cm-2 (Kalberla et al. 2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 6.8 x 10-11 (1.1 x 10-10) erg cm-2 count-1. If the light curve continues to decay with a power-law decay index of 1.20, the count rate at T+24 hours will be 0.0040 count s-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.7 x 10-13 (4.4 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/00361831. This circular is an official product of the Swift-XRT team.