TITLE: GCN CIRCULAR NUMBER: 5750 SUBJECT: Swift XRT boresight update DATE: 06/10/25 16:22:39 GMT FROM: David Burrows at PSU/Swift D. N. Burrows (PSU), A. Moretti (INAF-OAB), M. Perri (ASDC), M. Capalbi (ASDC), L. Angelini (NASA/GSFC) and J. E. Hill (USRA) report on behalf of the XRT instrument team: Approximately one month ago the XRT team reported a change in the XRT boresight since about 1 May 2006 that resulted in decreased positional accuracy (Burrows et al., GCN Circ. 5628). We have now completed preliminary analysis of the XRT pointing accuracy over the mission life and have derived a new boresight appropriate to current conditions. The XRT team will immediately begin reporting refined positions based on ground-processed data using the new boresight, as part of our beta-testing process. This will return XRT refined position accuracy to its previous level (typically 3.5 - 4 arcseconds, 90% radius.) Note that on-board positions have not yet been corrected to the new boresight. We will notify the community when this is done, but for the time being, on-board positions are several arcseconds away from the actual location, and the error circle included in automated XRT Position Notices should be increased by about 50% to account for this systematic error. The new boresight information will be included in an updated set of TELDEF files covering different time periods (part of the Swift CALDB), which will be made available to the rest of the community as part of the next software release, currently scheduled for no earlier than 27 November, 2006.