{
  "bibcode": "2006GCN..5628....1B",
  "body": "D. N. Burrows (PSU), J. E. Hill (GSFC/USRA), L. Angelini (NASA-GSFC), A. \nMoretti (INAF-OAB), A. P. Beardmore (U. Leicester), and J. A. Nousek (PSU) \nreport on behalf of the Swift/XRT team:\n\nWe wish to alert the community to an apparent change in the XRT boresight \nthat affects XRT source positions.  Comparison between XRT and optical \npositions for GRBs with optical counterparts since January 2006 shows that \nthe XRT position errors have increased since the beginning of May \n2006.  Analysis of bursts since 1 May 2006 shows that nearly 1/2 fall \noutside the quoted XRT 90% confidence radius (in most cases, only slightly \noutside the error circle), compared with ~6% for bursts before 1 May \n2006.  A plot of the ratio of the position error to the error circle radius \nshows a jump in early May.  This jump is presumed to be caused by a small \nshift in the XRT boresight.  The cause of the shift is unknown and is under \ninvestigation by the XRT team.  The XRT team is also in the process of \nrecalibrating the boresight to correct for this shift.  In the meantime, we \nare increasing the radii of our error circles by 50% to account for the \nsystematic error introduced by this boresight change.  Once we have \nrecalibrated the XRT boresight, we will reduce our error circle radii back \nto an appropriate level.",
  "circularId": 5628,
  "createdOn": 1159368194000,
  "email": "dxb15@psu.edu",
  "subject": "Swift XRT boresight",
  "submitter": "David Burrows at PSU/Swift  <dxb15@psu.edu>"
}