GCN Circular 5517
D. Grupe (PSU), O. Godet (U Leicester), and S. Barthelmy (GSFC) report on behalf of the Swift/XRT team We have analyzed the first 3 orbits with a total observing time of 5.3 ks of Swift XRT data of GRB 060904B (Grupe et al., GCN 5505), with 4.9 ks in photon counting mode (pc) and 427s in Windowed Timing mode (WT). The Photon Counting mode image provides a refined XRT position: RA(J2000) = 03h 52m 50.26s, Dec(J2000) = -00d 43' 32.5" with an error of 3.6" (90% confidence). This position is 3.9" away from the preliminary XRT position reported in GCN 5505. After the start of the XRT observation in pc mode, a giant flare started at 86s after the trigger and lasted to 490s after the trigger. This flare has a total 0.3-10.0 keV flux of 2.2e-9 ergs/s/cm2 and a fluence of 8.8e-7 ergs/cm2. The underlying afterglow decay slope is 1.11+/-0.25. The predicted flux 24h after the burst is 3e-13 ergs/cm2/s. Spectral fits to the Windowed Timing mode data during the flare suggest a significant absorption column density in excess of the Galactic value (1.21e21 cm**-2). The spectrum can be fitted by a single power law with a photon index Gamma=2.16+/-0.04 and an absorption column density NH=4.09+/-0.13 e21. Using the redshift z=0.703 as reported by Fugazza et al. (GCN 5513) the intrinsic absorption column density is NH,intr=7.6+/-0.4 e21 cm-2. The spectrum of the pc mode data agrees within the errors with this result. This circular is an official product of the Swift XRT Team.