[vsnet-grb-info 3258] GRB060502B: improvement in the XRT afterglow position

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5093
SUBJECT: GRB060502B: improvement in the XRT afterglow position
DATE:    06/05/09 08:16:10 GMT
FROM:    Eleonora Troja at INAF-IASFPA  <nora at ifc.inaf.it>

E. Troja (INAF-IASFPA), D. N. Burrows (PSU), and N. Gehrels (NASA/GSFC) 
report on behalf of the Swift XRT Team:


We have improved the astrometry of the Swift XRT observation of the
short hard burst GRB 060502B (Troja et al. 2006, GCN Circs. 5055, 5059,
and 5063) using 28 serendipitous X-ray sources detected in the field
during our 30 ks observation.  
We find 13 possible associations in the 2MASS or USNO 1.0B catalogs.
Four of these could be random matches, based on simulations with 
28 sources placed randomly within the XRT field of view.  From the 13 
possible associations, we selected 7 guide sources with offsets <~ 3 
arcsec from the X-ray sources, and used them to derive an astrometry
correction using the ccorr command in XIMAGE.  We obtained shifts in the
coordinates of DeltaRA=1.3 arcsec, DeltaDec=-0.6 arcsec.  

The final best position of the X-ray afterglow is:

RA(J2000) =   18h 35m 45.74s
Dec(J2000) = +52d 37' 52.47''

The uncertainty in the afterglow position on the detector is 2.0 arcsec.
The RMS scatter in the offsets between the corrected X-ray source
positions and the optical counterparts is 1.9 arcsec, giving us a 90%
confidence error radius of 4.4 arcsec.  
This position is 1.0 arcsec from the previously reported XRT refined
position (GCN 5063) and 3.1 arcsec from the candidate galaxy G1 (Price
et al. 2006, GCN 5071).