GCN Circular 6738
K.L. Page, P.A. Evans (U. Leicester), M.M Chester (PSU) and J.R. Cummings (NASA/UMBC) report on behalf of the Swift-XRT team: We have analysed the first four orbits of Swift-XRT data obtained for GRB 070810 (Cummings et al., GCN Circ. 6733), totalling 7.9 ks of Photon Counting (PC) mode data. The enhanced XRT position was given by Evans et al. in GCN Circ. 6736, and is 1.7 arcsec from the fading UVOT source (Chester et al., GCN Circ. 6735). The X-ray light-curve is almost flat for the first orbit of PC data, with alpha = 0.30 +0.18/-0.20. About 1.1 ks after the trigger, the decay steepened to a slope of 1.1 +/- 0.2. Fitting the PC data from the second to fourth orbits (i.e. after the break; 3.8-17.8 ks after the trigger), the spectrum can be modelled by an absorbed power-law with Gamma = 2.3 +/- 0.3 and NH = (1.3 +/- 0.5)e21 cm^-2, in excess of the Galactic value of 1.75e20 cm^-2. The 0.3-10 keV observed (unabsorbed) flux during this time is 2.17e-12 (3.34e-12) erg cm^-2 s^-1. If the decay continues with a slope of alpha ~ 1.1, the predicted count rate at 24 hours is 4.1e-3 count s^-1. Using a flux conversion factor (determined from the post-break spectrum described above) of 1 count s^-1 ~ 4e-11 erg cm^-2 s^-1 (observed) and 5.8e-11 erg cm^-2 s^-1 (unabsorbed), this corresponds to an observed (unabsorbed) flux of 1.6e-13 (2.4e-13) erg cm^-2 s^-1. This circular is an official product of the Swift-XRT team.