GCN Circular 3837
K.L. Page, A.P. Beardmore, M.R. Goad (U. Leicester), J.A. Kennea, D.N. Burrows (PSU), F. Marshall (GSFC) and A. Smale (NASA HQ) report on behalf of the Swift/XRT team: We have analysed the first 6 orbits of XRT data for GRB050820, spanning 88 - 3e5 seconds after the burst. There is a bright, fading afterglow in the field of view, with refined coordinates of RA(J2000): 22:29:38.2 Dec(J2000): 19:33:31.1 with an uncertainty of 7 arcseconds radius (90% containment). This is 5.9 arcsec from the on-board XRT position given in GCN 3830 and 6.1 arcsec from the optical afterglow reported in GCN 3829. XRT observations began in Windowed Timing mode 88 seconds after the trigger, followed by Photon Counting (PC) data from the start of the second orbit (4660 seconds after the burst). The PC data show a smooth decline with a decay slope of alpha = 1.13 +/- 0.04. The PC spectrum can be well fitted by a power-law with Gamma = 1.94 +/- 0.07, and an excess absorbing column of ~6e21 cm^-2 in the rest-frame of the GRB (taking the redshift of 2.612 from GCN 3833). Assuming a smooth, unbroken decay, the predicted count-rate at 24 hours (06.30 UT on 2005-08-21) is 0.085 count s^-1, corresponding to an unabsorbed 0.3-10 keV flux of 3.4e-12 erg cm^-2 s^-1.