TITLE: GCN CIRCULAR NUMBER: 9790 SUBJECT: GRB 090813: Swift-XRT refined analysis DATE: 09/08/13 15:43:56 GMT FROM: Andy Beardmore at U Leicester A. P. Beardmore (U. Leicester) and J. R. Cummings (NASA/UMBC) report on behalf of the Swift-XRT team: We have analysed 8.8 ks of XRT data for GRB 090813 (Cummings et al., GCN Circ. 9781), from 84 s to 19.3 ks after the BAT trigger. The data span four orbits, comprising 810 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Osborne et al. (GCN. Circ 9785). The X-ray light curve initially follows a shallow decay, with a decay index of 0.22 +/ 0.10, before steepening to an index of 1.15 +/- 0.03 at about 450 s after the trigger. A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.10 +/- 0.07. The best-fitting absorption column is (2.23 +0.20 -0.19) x 10^21 cm^-2, in excess of the Galactic value of 6.3 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index of 2.00 +0.11 -0.10 and a best-fitting absorption column of (2.3 +/- 0.3) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.3 x 10^-11 (6.4 x 10^-11) erg cm^-2 count^-1. If the light curve continues to decay with a power-law decay index of 1.15, the estimated count rate at T+24 hours will be 0.016 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.9 x 10^-13 (1.0x 10^-12) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00359884. This circular is an official product of the Swift-XRT team.