TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3395 SUBJECT: GRB 050509B: Swift/XRT refined analysis DATE: 05/05/09 17:10:13 GMT FROM: David Burrows at PSU/Swift E. Rol, K. Page (Univ. of Leicester) D. N. Burrows (Penn State), N. Gehrels (GSFC), M. Goad, C. Hurkett (Univ. of Leicester), J. Kennea (Penn State), P O'Brien (Univ. of Leicester) report on behalf of the Swift XRT team: We have analyzed the Swift XRT data from the short/hard burst 050509B (Hurkett et al. 2005, GCN 3381) using 3 orbits with a total exposure time of 6648 seconds in Photon Counting (PC) mode, starting 62 seconds after the trigger. The refined coordinates of the X-ray afterglow are: RA(J2000) = 12:36:13.9 Dec = +28:59:01 with an estimated error of 8 arcseconds (90% containment). Note that we have increased the estimate of the error circle radius to account for systematic variations in the centroid position for this source, which has very few counts. The source is detected in the first 400 seconds of PC mode at a count rate of 0.03 counts/sec, after which it disappears below background level, and is not seen in the data from later orbits, with an estimated decay slope steeper than -1. Assuming a Crab-like spectrum, we estimate an average flux level of 1e-12 erg/cm^2/s (0.3 - 10 keV), between 62 and 262 seconds after the burst trigger. This is the faintest X-ray afterglow yet detected by Swift/XRT at such an early time.