{
  "bibcode": "2006GCN..5773....1R",
  "body": "J. L. Racusin, D. N. Burrows (PSU), and N. Gehrels (NASA/GSFC) report on \nbehalf of the Swift XRT team:\n\nWe have re-analyzed the full XRT data set of the short hard GRB 060801. \nXRT observed the field of GRB 060801 for a total of 75 ks between August 1 \nand August 5, 2006 in Photon Counting mode in order to obtain a more \naccurate position of the short-lived X-ray afterglow.\n\nIn the full data set we find 23 serendipitous X-ray sources detected with \nthe XIMAGE detect algorithm with S/N > 3, 12 of which have near-by optical \ncounterparts in the SDSS.  We match these sources to obtain a best fit \nmean frame shift, carefully accounting for several instrumental factors \ndescribed below.\n\nAt the time of these observations there was a problem with the bias maps \nthat caused spurious hot pixels to remain in the cleaned event lists after \nstandard processing.  To remove this contamination, we filtered out events \nwith energies < 0.4 keV.  For more information on the bias map issue, see \nhttp://www.swift.ac.uk/xrtdigest.shtml.\n\nWe also apply an exposure map correction to the images before input into \nXRTCENTROID to account for any sources landing on or near the bad columns \nin the XRT CCD. In the first orbit, the GRB did in fact lie on one of \nthese columns.  We use XRTCENTROID to calculate the positions of the GRB \nand the serendipitous sources.\n\nAdditional position error arises from the Swift star tracker solution \nuncertainty.  This error is small once the spacecraft has completely \nsettled after a slew, but is not achieved until ~100s after the XRT \nobservations begin.  Therefore, we exclude the first 100s of each orbit, \nreducing the total used exposure used time to 69 ks.\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 (Racusin et al., GCN 5382), of:\n\ndRA: -3.60\"   dDec: -0.90\"  +/- 0.64\"\n\nand a new XRT refined position of:\n\nRA(J2000):   14h 12m 01.31s\nDec(J2000): +16d 58' 54.0\"\n\nwith an estimated uncertainty of 2.1 arcseconds (90% containment).\n\nThis position is 4.2 arcseconds from the refined XRT position given in \nRacusin et al. (GCN 5382), 1.9 arcseconds from the boresight corrected XRT \nposition using the new TELDEF described by Burrows et al. (GCN 5750), and \n0.5 arcseconds from the frame shifted position given in Butler et al. (GCN \n5389) based upon only the first 25 ks of data.  Potential host galaxies B \n& D from Castro-Tirado et al. and Piranomonte et al. (GCNs 5384 & 5386) \nboth lie inside the new XRT refined error circle.  A figure comparing all \nof these positions is available at: \nhttp://www.swift.psu.edu/images/grb060801_astrometry.gif\n\nGalaxy B has a redshift of 1.131 as given by Cucchiara et al. (GCN 5460). \nWe encourage observations of galaxy D to obtain its redshift.\n\nThis Circular is an official product of the Swift XRT team.",
  "circularId": 5773,
  "createdOn": 1162225813000,
  "email": "racusin@astro.psu.edu",
  "subject": "GRB 060801: Swift/XRT Astrometry Correction",
  "submitter": "Judith Racusin at PSU  <racusin@astro.psu.edu>",
  "eventId": "GRB 060801"
}