Refined Astrometry and Positions for 179 Swift X-ray Afterglows

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Abstract
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 to November 1, 2006. The positions are determined by detecting X-ray field sources in the deep X-ray images and comparing the centroids to those of optical sources in the Digitized Sky Survey (DSS) red2 catalog or the Sloan Digital Sky Survey (SDSS) DR-5 catalog. Half of the 90% confidence error region radii are 20 ksec) and cannot be generated nearly as rapidly after the GRB. The positions derived for >90% of 77 bursts with optical afterglows are consistent with the optical transient positions, without the need for systematic error. About 20% of the afterglows positions require a sizable shift in the Swift satellite aspect. We discuss the optical/X-ray properties of the field sources and discuss implications of the frame offsets for studies of optically dark GRBs.

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