TITLE: GCN CIRCULAR NUMBER: 8261 SUBJECT: GRB 080916C: Swift XRT refined analysis DATE: 08/09/17 15:24:55 GMT FROM: Matteo Perri at ISAC/ASDC M. Perri, B. Preger, G. Stratta (ASDC) report on behalf of the Swift XRT team: The Swift XRT started observing the field of the FERMI GRB 080916C (Goldstein et al., GCN Circ. 8245) at 2008-09-16 17:11:28 UT, about 17 hours after the trigger. We confirm that the X-ray source reported by Kennea (GCN Circ. 8253) is the afterglow of the GRB. Using 2614 s of XRT Photon Counting mode data and 2 UVOT images, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching to the USNO-B1 catalogue): RA, Dec = 119.84684, -56.63801 which is equivalent to: RA (J2000): 07 59 23.24 Dec (J2000): -56 38 16.8 with an uncertainty of 1.9 arcsec (90% confidence). This position is consistent with the XRT position reported by Evans et al. (GCN Circ. 8255) using less data. The X-ray light curve from T+61 ks to T+102 ks can be fit with a single power-law model with a decay index of 1.7 (-0.7) (+1.0). A 7.6 ks exposure X-ray spectrum from T+61 ks to T+102 ks can be well fit by an absorbed power-law model with a photon index of 2.1 (+0.9 -0.7) and a column density of 3.7 (+3.3) (-2.1) x 10^21 cm^-2. We note that the Galactic column density value in the direction of the burst is 1.5 x 10^21 cm^-2. The observed 0.3-10.0 keV flux is 8.6 x 10-13 erg cm^-2 s^-1 which corresponds to an unabsorbed flux of 1.5 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 3 x 10^-13 erg cm^-2 s^-1 at T+2 days. This circular is an official product of the Swift-XRT team.