GCN Circular 4313
L. Angelini (GSFC), J. E. Hill (USRA/GSFC), A. Moretti, G. Chincarini (OAB), D. N. Burrows, J. A. Kennea, J. Racusin (PSU), and J. Osborne (U. Leicester) report on behalf of the Swift XRT team: A new TELDEF file has been implemented in the Swift Science Data Center (SDC) pipeline processing for the XRT data. This new file implements a correction for the XRT boresight (see Moretti et al. 2005, astroph/0511604; also see J. Hill 2005, http://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/swift/docs/xrt/SWIFT-XRT-CALDB-06_v2.pdf), shifting it by about 4 arcseconds from the previous boresight. The result is a more accurate assignment of sky positions to all XRT photons, which improves the average XRT 90% confidence error circle radius for source detections by a factor of two, from about 6.5 arcseconds to between 3 and 4 arcseconds (note that the error circle radius for a particular target depends on the photon statistics for that target). All data processed at the SDC after 15:00 Eastern Standard Time on 2 December 2005 use the new boresight definition. CALDB has been updated and the new TELDEF is included in the latest tar file available at ftp://legacy.gsfc.nasa.gov/caldb/data/swift/xrt/goodfiles_swift_xrt.tar.Z This CALDB release is tagged 20051201. A similar change will be made on-board to the parameters used to determine on-board source centroids in the near future. We will send out a GCN Circular announcing that change once it has been implemented.