Positions for 179 Swift X-ray afterglows

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 red2 catalog or the Sloan Digital Sky Survey Data Release 5 catalog. Half of the 90% confidence error region radii are <2.2".

The error regions areas are typically ~4 times smaller than the best XRT team error regions, although the positions require deep X-ray integration (>20ks) 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 afterglow positions require a sizable shift in the Swift satellite aspect.