TITLE: GCN CIRCULAR NUMBER: 9876 SUBJECT: GRB090902B: Swift XRT refined analysis DATE: 09/09/03 15:30:29 GMT FROM: Giulia Stratta at ASDC G. Stratta, V. D'Elia, M. Perri (ASDC) report on behalf of the Swift XRT team: At 23:36 UT, September 2nd, 2009, Swift began a Target of Opportunity observation of the Fermi GBM/LAT discovered burst GRB 090902B (de Palma et al., GCN 9867, Bissaldi et al., GCN 9866) about 12.5 hours after the trigger. We confirm that the X-ray source reported by Kennea et al. (GCN Circ. 9868) shows a decaying behavior and is likely the afterglow of GRB 090902B. The enhanced XRT position for this burst was given by Evans (GCN. Circ 9871). The X-ray light curve from T+12.53 hr to T+17.38 hr can be fit with a single power-law model with a decay index of -1.3+/-0.9 (90% confidence level). With 4.8 ks of integration time, the X-ray spectrum from T+12.53 hr to T+17.38 hr after the Fermi/GBM trigger can be fit by an absorbed power-law model with a photon index of 2.1 (+0.3,-0.3) and a rest frame column density of (3.4+/-0.9) x 10^22 cm^-2 at z=1.822 (Cucchiara et al., GCN 9873) in addition to the Galactic column density in the direction of the burst (3.88 x 10^20 cm^-2, Kalberla et al. 2005). Errors are at 90% confidence level. The observed 0.3-10.0 keV flux is 3.2 x 10^-12 erg cm^-2 s^-1 which corresponds to an unabsorbed flux of 4.1 x 10^-12 erg cm^-2 s^-1. Providing the source continues to decay at the same rate, we predict an observed flux of about 8.7 x 10^-13 erg cm^-2 s^-1 at T+2 days. This circular is an official product of the Swift-XRT team. [GCN OPS NOTE(03sep09): Per author's request, the missing Cucchiara reference was added.]