{
  "bibcode": "2005GCN..3494....1B",
  "body": "D. N. Burrows (PSU), S. Patel (MSFC), C. Sarazin (U. Virginia), E. Rol, M. \nR. Goad, P. T. O'Brien, R. Willingale (U. Leicester), and N. Gehrels (GSFC) \nreport:\n\nMotivated by Bloom et al. (astro-ph 0505480), the Swift XRT team has \nreviewed our analysis of the XRT position for GRB 050509b (Rol et al., GCN \n3395).  We have taken into account the low counting statistics, cluster \nemission in the field and astrometric corrections.  In order to evaluate \npossible cluster contributions, we\nhave tested a variety of aperture sizes and time intervals.  We find that\nthe cluster emission contributes no more than 1 photon (on average) to the\ncounts detected within any of our temporal and spatial regions, and is \ntherefore\nunlikely to bias the results.   We use the entire first orbit of data to\nmaximize the signal to noise, and detect 11 photons in a 15 arcsecond (radius)\nsource region.  The xrtcentroid tool in the XRTDAS software package \ncalculates the following position for these photons: RA(J2000) = \n12:36:13.80, Dec(J2000) = 28:59:01.0\nin the Swift frame of reference (shifted by 0.1 second in RA from the position\nreported in Rol et al.).\n\nWe have corrected the Swift position to the 2MASS system astrometry by \nregistering\nour 50 ks Chandra image to 2MASS coordinates using sources appearing in both,\nand then registering the 30 ks XRT observation to the Chandra image.  This\nastrometric correction gives a shift of -2.9 arcseconds in RA and +0.3\narcseconds in declination, for a final XRT afterglow position of:\n\n  RA(J2000) = 12h 36m 13.58s, Dec(J2000) = 28d 59' 01.3\".\n\nThis position is 9.8 arcseconds from the center of the E1 galaxy. Combining\npre-launch calibration data with the uncertainties in the astrometric\ncorrection, we estimate an error circle radius of about 9.3 arcseconds (90%\ncontainment).  This error circle is dominated by the Poisson statistics \nassociated with\nthe low number of source counts.",
  "circularId": 3494,
  "createdOn": 1117241218000,
  "email": "dxb15@psu.edu",
  "subject": "GRB 050509b: refined XRT/Chandra afterglow position analysis",
  "submitter": "David Burrows at PSU/Swift  <dxb15@psu.edu>",
  "eventId": "GRB 050509B"
}