TITLE: GCN CIRCULAR NUMBER: 5785 SUBJECT: GRB 061028: Swift/XRT Astrometry Correction DATE: 06/11/02 16:30:42 GMT FROM: Judith Racusin at PSU J. L. Racusin (PSU), D. N. Burrows (PSU), T. Mineo (INAF-IASFPA), T. Sakamoto (NASA/ORAU), and N. Gehrels (NASA/GSFC) report on behalf of the Swift XRT team: We have re-analyzed the full XRT data set of GRB 061028. XRT observed the field of GRB 061028 for a total exposure time of 49 ks between October 28 and October 29, 2006 in Photon Counting mode. To further improve the accuracy of the previously reported position (Cusumano et al., GCN 5767), we performed an astrometry correction using 46 ks of the total exposure time (when the satellite position was stable). In this data set we find 18 serendipitous X-ray sources detected with the XIMAGE detect algorithm with S/N > 3, 7 of which have near-by optical counterparts in the USNO-B1 catalog. We match these sources to obtain a best fit mean frame shift, carefully accounting for several instrumental factors including exposure map correction, and additional hot pixel removal. We calculate the statistical position errors using the empirical fits as described in Moretti et al. (2006, A&A, 448, L9), assuming that the astrometric correction removes the 3.5" systematic error normally applied to XRT positions to account for errors in the star tracker attitude solution. The result of this analysis leads to a mean frame shift from the previously reported position (Cusumano et al., GCN 5767), of: RA offset: +0.15s +/- 0.04s Dec offset: +1.3" +/- 0.7" and a new XRT astrometry corrected position of: RA(J2000): 06h 28m 54.66s Dec(J2000): +46d 17' 57.0" with an estimated uncertainty of 2.2 arcseconds (radius, 90% containment). This position is 2.2 arcseconds from the refined XRT position given in Cusumano et al. (GCN 5767), 0.7 arcseconds from the position measured by Butler et al. (astro-ph/0611031), 1.5 arcseconds from the optical object described in Cenko et al. (GCN 5770), and 4.0 arcseconds from the optical object described in Bloom et al. (GCN 5768). A figure comparing all of these positions is available at: http://www.swift.psu.edu/images/grb061028_astrometry.gif This Circular is an official product of the Swift XRT team.