J. L. Racusin (PSU), D. N. Burrows (PSU), T. Mineo (INAF-IASFPA), T.
Sakamoto (NASA/ORAU), and N. Gehrels (NASA/GSFC) report on behalf of the
Swift XRT team:
We have re-analyzed the full XRT data set of GRB 061028. XRT observed the
field of GRB 061028 for a total exposure time of 49 ks between October 28
and October 29, 2006 in Photon Counting mode.
To further improve the accuracy of the previously reported position
(Cusumano et al., GCN 5767), we performed an astrometry correction using
46 ks of the total exposure time (when the satellite position was stable).
In this data set we find 18 serendipitous X-ray sources detected with the
XIMAGE detect algorithm with S/N > 3, 7 of which have near-by optical
counterparts in the USNO-B1 catalog. We match these sources to obtain a
best fit mean frame shift, carefully accounting for several instrumental
factors including exposure map correction, and additional hot pixel
We calculate the statistical position errors using the empirical fits as
described in Moretti et al. (2006, A&A, 448, L9), assuming that the
astrometric correction removes the 3.5" systematic error normally applied
to XRT positions to account for errors in the star tracker attitude
The result of this analysis leads to a mean frame shift from the
previously reported position (Cusumano et al., GCN 5767), of:
RA offset: +0.15s +/- 0.04s Dec offset: +1.3" +/- 0.7"
and a new XRT astrometry corrected position of:
RA(J2000): 06h 28m 54.66s
Dec(J2000): +46d 17' 57.0"
with an estimated uncertainty of 2.2 arcseconds (radius, 90% containment).
This position is 2.2 arcseconds from the refined XRT position given in
Cusumano et al. (GCN 5767), 0.7 arcseconds from the position measured by
Butler et al. (astro-ph/0611031), 1.5 arcseconds from the optical object
described in Cenko et al. (GCN 5770), and 4.0 arcseconds from the optical
object described in Bloom et al. (GCN 5768).
A figure comparing all of these positions is available at:
This Circular is an official product of the Swift XRT team.