TITLE:   GCN CIRCULAR
NUMBER:  5872
SUBJECT: Swift XRT on-board boresight update 
DATE:    06/11/27 16:43:13 GMT
FROM:    Jamie A. Kennea at PSU/Swift-XRT  <kennea@astro.psu.edu>

J. A. Kennea, 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:

As previously reported there has been a change in the XRT boresight since 
about 1 May 2006 that resulted in decreased positional accuracy (Burrows 
et al., GCN Circ. 5628, GCN Circ. 5750). On November 24th, 2006 the XRT 
team updated the onboard boresight definition used to calculate automatic 
burst positions to correct for this change. As a result of this update the 
on-board calculated position and position error reported in the XRT 
Position Notices are now correct. We note that this change does not 
correct positions derived from ground processed data, which will require 
an update to the CALDB TELDEF file. This is due to be released to the 
community with the next Swift software update. Note that the XRT Team are 
currently utilizing a pre-release of this new TELDEF file when reporting 
positions in GCN Notices and Reports.