TITLE: GCN CIRCULAR NUMBER: 8226 SUBJECT: GRB 080913: Swift-XRT refined analysis DATE: 08/09/13 17:30:12 GMT FROM: Andy Beardmore at U Leicester A.P. Beardmore (U. Leicester) and P. Schady (MSSL-UCL) report on behalf of the Swift-XRT team : The Swift-XRT started observing the field of GRB 080913 (trigger number 324561, Schady et al., GCN Circ. 8217) at 2008-09-13 06:48:30 UT, 94 s after the trigger. The best XRT position is the UVOT-enhanced position reported by Beardmore et al. (GCN Circ. 8219). The X-ray light curve presently spans 5 orbits of photon counting mode data from T+108 s to T+23 ks. The light curve shows a number of small flares in the first orbit, with the largest giving a factor of ~7 increase in count rate at T+1.8 ks, on top of a power-law decay of index 1.20 +0.16 -0.13. A 2.7 ks exposure X-ray spectrum from T+108 s to T+7.6 ks can be well fit by an absorbed power-law with a photon index of 1.69 +0.46 -0.41 and a column density consistent with the Galactic value of 3.2 x 10^20 cm^-2 in the direction of the burst. The observed 0.3-10.0 keV flux is (3.2 +0.9 -1.6) x 10^-12 erg cm^-2 s^-1 which corresponds to an unabsorbed flux of (3.7 +1.6 -1.7) x 10^-12 erg cm^-2 s^-1. The count to flux conversion factor is 4.6 x 10^-11 erg cm^-2 count^-1. Providing the source continues to decay at the same rate, we predict a count rate of 2.8 x 10^-4 count s^-1 at T+1 day. This circular is an official product of the Swift-XRT team.