GCN Circular 5750

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
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 ).  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.