TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3752 SUBJECT: GRB 050803: early Swift XRT analysis results DATE: 05/08/04 03:50:32 GMT FROM: Judith Racusin at PSU J. Racusin (PSU), C. Pagani (INAF-OAB), D. Burrows (PSU), K. Page (U. Leicester), on behalf on the Swift XRT team: We have analyzed the first 3 orbits of Swift XRT data from GRB 050803 (Band et al., GCN 3748). The refined coordinates from the downlinked ground processed data of the X-ray afterglow are: RA(J2000): 23h 22m 37.8s Dec(J2000): +5d 47' 5.5" This position is 60 arcseconds from the BAT position given in GCN 3748 (Band et al.), and 2.1 arcseconds from the on-board determined XRT position reported in GCN 3748. We estimate an uncertainty of 6 arcseconds radius (90% containment). Data in Window Timing (WT) mode began at 19:16:40 UT, 160 seconds after the BAT trigger, and continued for 84s before switching into Photon Counting (PC) mode. The 0.3-10 keV afterglow light curve shows a rapidly fading X-ray source which can be fitted with a power law of slope alpha=-5.4+/-0.1 up to a break around 290 s, after which the light curve flattens where it can be fitted with a power law of slope alpha=-0.53+/-0.02 as far as current data extends. The predicted flux at 24 hours after the trigger is 2.96E-12 erg cm^-2 s^-1. A preliminary spectral fit to this data gives a spectral power law photon index of 1.8 +/- 0.1 with nH=(19.8+/-3.2)E20 cm^-2. This is significantly higher than the Galactic nH along the line of sight which is 5.6E+20 cm^-2.