GCN Circular 3494

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
Not found
 
 
).  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.