TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5093 SUBJECT: GRB060502B: improvement in the XRT afterglow position DATE: 06/05/09 08:16:10 GMT FROM: Eleonora Troja at INAF-IASFPA E. Troja (INAF-IASFPA), D. N. Burrows (PSU), and N. Gehrels (NASA/GSFC) report on behalf of the Swift XRT Team: We have improved the astrometry of the Swift XRT observation of the short hard burst GRB 060502B (Troja et al. 2006, GCN Circs. 5055, 5059, and 5063) using 28 serendipitous X-ray sources detected in the field during our 30 ks observation. We find 13 possible associations in the 2MASS or USNO 1.0B catalogs. Four of these could be random matches, based on simulations with 28 sources placed randomly within the XRT field of view. From the 13 possible associations, we selected 7 guide sources with offsets <~ 3 arcsec from the X-ray sources, and used them to derive an astrometry correction using the ccorr command in XIMAGE. We obtained shifts in the coordinates of DeltaRA=1.3 arcsec, DeltaDec=-0.6 arcsec. The final best position of the X-ray afterglow is: RA(J2000) = 18h 35m 45.74s Dec(J2000) = +52d 37' 52.47'' The uncertainty in the afterglow position on the detector is 2.0 arcsec. The RMS scatter in the offsets between the corrected X-ray source positions and the optical counterparts is 1.9 arcsec, giving us a 90% confidence error radius of 4.4 arcsec. This position is 1.0 arcsec from the previously reported XRT refined position (GCN 5063) and 3.1 arcsec from the candidate galaxy G1 (Price et al. 2006, GCN 5071).