Refined Astrometry and Positions for 179SwiftX-Ray Afterglows

We present a refined catalog for the positions of 179 Gamma-ray burst (GRB)X-ray afterglows observed by the X-ray Telescope (XRT) on Swift prior toNovember 1, 2006. The positions are determined by detecting X-ray field sourcesin the deep X-ray images and comparing the centroids to those of opticalsources in the Digitized Sky Survey (DSS) red2 catalog or the Sloan Digital SkySurvey (SDSS) DR-5 catalog. Half of the 90% confidence error region radii are<2.2 arcseconds. The error regions areas are typically ~4 times smaller thanthe best XRT-team error regions, although the positions require deep X-rayintegration (>20 ksec) and cannot be generated nearly as rapidly after the GRB.The positions derived for >90% of 77 bursts with optical afterglows areconsistent with the optical transient positions, without the need forsystematic error. About 20% of the afterglows positions require a sizable shiftin the Swift satellite aspect. We discuss the optical/X-ray properties of thefield sources and discuss implications of the frame offsets for studies ofoptically dark GRBs.Comment: 8 pages, 5 figures, 3 tables, Accepted to A