TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3494 SUBJECT: GRB 050509b: refined XRT/Chandra afterglow position analysis DATE: 05/05/28 00:46:58 GMT FROM: David Burrows at PSU/Swift D. N. Burrows (PSU), S. Patel (MSFC), C. Sarazin (U. Virginia), E. Rol, M. R. Goad, P. T. O'Brien, R. Willingale (U. Leicester), and N. Gehrels (GSFC) report: Motivated by Bloom et al. (astro-ph 0505480), the Swift XRT team has reviewed our analysis of the XRT position for GRB 050509b (Rol et al., GCN 3395). We have taken into account the low counting statistics, cluster emission in the field and astrometric corrections. In order to evaluate possible cluster contributions, we have tested a variety of aperture sizes and time intervals. We find that the cluster emission contributes no more than 1 photon (on average) to the counts detected within any of our temporal and spatial regions, and is therefore unlikely to bias the results. We use the entire first orbit of data to maximize the signal to noise, and detect 11 photons in a 15 arcsecond (radius) source region. The xrtcentroid tool in the XRTDAS software package calculates the following position for these photons: RA(J2000) = 12:36:13.80, Dec(J2000) = 28:59:01.0 in the Swift frame of reference (shifted by 0.1 second in RA from the position reported in Rol et al.). We have corrected the Swift position to the 2MASS system astrometry by registering our 50 ks Chandra image to 2MASS coordinates using sources appearing in both, and then registering the 30 ks XRT observation to the Chandra image. This astrometric correction gives a shift of -2.9 arcseconds in RA and +0.3 arcseconds in declination, for a final XRT afterglow position of: RA(J2000) = 12h 36m 13.58s, Dec(J2000) = 28d 59' 01.3". This position is 9.8 arcseconds from the center of the E1 galaxy. Combining pre-launch calibration data with the uncertainties in the astrometric correction, we estimate an error circle radius of about 9.3 arcseconds (90% containment). This error circle is dominated by the Poisson statistics associated with the low number of source counts.