{
  "bibcode": "2007GCN..6726....1O",
  "body": "J. P. Osborne (U Leicester), M. R. Goad (U Leicester), P. A. Evans (U \nLeicester), A. P. Beardmore (U Leicester), L. G. Tyler (U Leicester),\nD. N. Burrows (PSU), N. Gehrels (GSFC) and S. D. Barthelmy (GSFC), report on \nbehalf of the Swift Team:\n\nThe Swift XRT team will from now be providing better positions earlier for \nmost GRBs by automatically issuing: 1) an early position update notice \nbased on ground processing; 2) a circular giving an enhanced XRT position \nand error derived from a rectification of the UVOT field with USNO B1. See \nhttp://www.swift.ac.uk/xrtpos.php.\n\n1) Position Update Notices\n\nGCN Swift XRT position update notices will soon be routinely distributed.\nThe position and error will be based upon fully automated ground processing\nof SPER data (TDRSS-downlinked Single Pixel Event Report data in Photon\nCounting mode), fit using a PSF model with the dead columns to give a\nmore reliable position than is possible on-board. This position update notice\nwill typically be sent within 15 minutes of the start of the XRT observation.\nRobotic telescopes may want to adapt their reaction strategy.\n\nSPER data come only during the first snap-shot observation of a burst,\nit is distributed to the XRT team at ~5 minute intervals; the notice will be \ngenerated from the combined first and second SPER datasets, as this offers the \nbest position improvement for the shortest delay (notice distribution will be \nforced if a second SPER is not received within 9 minutes of the first).  If this \nis not the first XRT_Position notice for a burst then the notice UPDATE_FLAG \nkeyword is set, the email subject line will have 'UPDATE' added, and in the \npackets to socket sites the 2^25 bit will be set in the PKT_MISC field \n(http://gcn.gsfc.nasa.gov/gcn/sock_pkt_def_doc.html).\n\nSubsequent SPER data may allow further improvement of the early position \ninformation. This will not be distributed by GCN notice, but will be \nimmediately available at http://www.swift.ac.uk/spertable.php.  A link from that \npage describes the derivation of positions from SPER data in detail.  Over 80% \nof bursts can have a position derived in this way; for these, the 90% position \nerror radius is <3.9\" half of the time.\n\n2) Enhanced Position Circulars\n\nGCN Swift XRT position circulars will soon be routinely distributed when a \nfirst UVOT-enhanced XRT position becomes available.  Use of the XRT-UVOT \nboresight alignment and the UVOT-USNO B1 astrometric registration for the \nbursts simultaneously covered by the XRT PC mode and UVOT V band gives more \nprecise and accurate XRT positions than those based on star tracker data \npreviously distributed.  Around 60% of XRT bursts can have their positions \nimproved by this method; such UVOT-enhanced XRT burst positions have a 90% \nconfidence error radius <2.1\" half of the time, and are typically available \nwithin 2-3 hours of the burst (Goad et al. A&A submitted, arXiv:0708.0986,\nhttp://www.swift.ac.uk/xrt_positions/Goad.pdf).  These rapidly available \nenhanced positions have an accuracy comparable to that obtainable in one to \nseveral days using serendipitous X-ray sources (eg Butler AJ 133, 1027, 2007).\n\nImprovement of a UVOT-enhanced XRT position is possible as more data become \navailable after the initial enhanced position circular.  Such subsequently \nimproved positions will not be announced via the GCN but will be immediately \navailable at http://www.swift.ac.uk/xrt_positions/index.php\n\nIt is not yet possible to enhance a larger fraction of positions due to the \nneed for simultaneous V band and XRT PC mode data when Swift is still and for \nat least 10 XRT counts. We hope to improve this in the future.",
  "circularId": 6726,
  "createdOn": 1186662762000,
  "email": "julo@star.le.ac.uk",
  "subject": "GCN Swift XRT update position notices and enhanced position circulars",
  "submitter": "Julian Osborne at U.of Leicester  <julo@star.le.ac.uk>"
}