TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4232 SUBJECT: GRB051109B: XRT refined analysis DATE: 05/11/09 18:42:43 GMT FROM: Gianpiero Tagliaferri at OAB-INAF G. Tagliaferri (INAF-OAB), S. Campana (INAF-OAB), P. Romano (INAF-OAB), T. Mineo (INAF-IASFPA), D.N. Burrows (PSU) report on behalf of the Swift/XRT team: We have analysed 4.3 ks of XRT observations of the afterglow of GRB051109B (Tagliaferri et al., GCN 4222; Campana et al. GCN4226). The X-ray light-curve from 100s after the burst up to about 10 ks after the burst is quite flat and well represented by a single power law with a decay index alpha=-0.55+/-0.08 (90% containment). The spectrum can be modeled with a power-law with photon index Gamma = 2.33+/-0.30 (90% containment). There is evidence for an absorbing column higher than the Galactic value (1.3e21 cm^-2) at a level of (3.1+/-1.2)e21 cm^-2. At 200 s the 0.5-10 keV unabsorbed flux was 1.7e-10 erg cm^-2 s^-1. At 10000 s the afterglow had faded to flux level of 2.0e-12 erg cm^-2 s^-1. Assuming the current flat decay we predict a flux of ~5.6e-13 erg cm^-2 s^-1 at 1 d from the burst. [GCN_OPS_NOTE(10nov05): Per author's request, the flux level in the last paragraph "2.0e-11" was changed to "2.0e-12".]