{
  "bibcode": "2006GCN..5785....1R",
  "body": "J. L. Racusin (PSU), D. N. Burrows (PSU), T. Mineo (INAF-IASFPA), T. \nSakamoto (NASA/ORAU), and N. Gehrels (NASA/GSFC) report on behalf of the \nSwift XRT team:\n\nWe have re-analyzed the full XRT data set of GRB 061028.  XRT observed the \nfield of GRB 061028 for a total exposure time of 49 ks between October 28 \nand October 29, 2006 in Photon Counting mode.\n\nTo further improve the accuracy of the previously reported position \n(Cusumano et al., GCN 5767), we performed an astrometry correction using \n46 ks of the total exposure time (when the satellite position was stable). \nIn this data set we find 18 serendipitous X-ray sources detected with the \nXIMAGE detect algorithm with S/N > 3, 7 of which have near-by optical \ncounterparts in the USNO-B1 catalog.  We match these sources to obtain a \nbest fit mean frame shift, carefully accounting for several instrumental \nfactors including exposure map correction, and additional hot pixel \nremoval.\n\nWe calculate the statistical position errors using the empirical fits as \ndescribed in Moretti et al. (2006, A&A, 448, L9), assuming that the \nastrometric correction removes the 3.5\" systematic error normally applied \nto XRT positions to account for errors in the star tracker attitude \nsolution.\n\nThe result of this analysis leads to a mean frame shift from the \npreviously reported position (Cusumano et al., GCN 5767), of:\n\nRA offset: +0.15s +/- 0.04s   Dec offset: +1.3\" +/- 0.7\"\n\nand a new XRT astrometry corrected position of:\n\nRA(J2000):   06h 28m 54.66s\nDec(J2000): +46d 17' 57.0\"\n\nwith an estimated uncertainty of 2.2 arcseconds (radius, 90% containment).\n\nThis position is 2.2 arcseconds from the refined XRT position given in \nCusumano et al. (GCN 5767), 0.7 arcseconds from the position measured by \nButler et al. (astro-ph/0611031), 1.5 arcseconds from the optical object \ndescribed in Cenko et al. (GCN 5770), and 4.0 arcseconds from the optical \nobject described in Bloom et al. (GCN 5768).\n\nA figure comparing all of these positions is available at: \nhttp://www.swift.psu.edu/images/grb061028_astrometry.gif\n\nThis Circular is an official product of the Swift XRT team.",
  "circularId": 5785,
  "createdOn": 1162485042000,
  "email": "racusin@astro.psu.edu",
  "subject": "GRB 061028: Swift/XRT Astrometry Correction",
  "submitter": "Judith Racusin at PSU  <racusin@astro.psu.edu>",
  "eventId": "GRB 061028"
}