[vsnet-grb-info 3794] Swift XRT boresight
GCN Circulars
gcncirc at capella.gsfc.nasa.gov
Wed Sep 27 23:43:19 JST 2006
TITLE: GCN GRB OBSERVATION REPORT
NUMBER: 5628
SUBJECT: Swift XRT boresight
DATE: 06/09/27 14:43:14 GMT
FROM: David Burrows at PSU/Swift <dxb15 at psu.edu>
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.
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