GCN Circular 5628
D. N. Burrows (PSU), J. E. Hill (GSFC/USRA), L. Angelini (NASA-GSFC), A. Moretti (INAF-OAB), A. P. Beardmore (U. Leicester), and J. A. Nousek (PSU) report on behalf of the Swift/XRT team: We wish to alert the community to an apparent change in the XRT boresight that affects XRT source positions. Comparison between XRT and optical positions for GRBs with optical counterparts since January 2006 shows that the XRT position errors have increased since the beginning of May 2006. Analysis of bursts since 1 May 2006 shows that nearly 1/2 fall outside the quoted XRT 90% confidence radius (in most cases, only slightly outside the error circle), compared with ~6% for bursts before 1 May 2006. A plot of the ratio of the position error to the error circle radius shows a jump in early May. This jump is presumed to be caused by a small shift in the XRT boresight. The cause of the shift is unknown and is under investigation by the XRT team. The XRT team is also in the process of recalibrating the boresight to correct for this shift. In the meantime, we are increasing the radii of our error circles by 50% to account for the systematic error introduced by this boresight change. Once we have recalibrated the XRT boresight, we will reduce our error circle radii back to an appropriate level.