[vsnet-grb-info 3966] GRB 061028: Swift/XRT Astrometry Correction

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  5785
SUBJECT: GRB 061028: Swift/XRT Astrometry Correction
DATE:    06/11/02 16:30:42 GMT
FROM:    Judith Racusin at PSU  <racusin at astro.psu.edu>

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 
removal.

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 
solution.

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: 
http://www.swift.psu.edu/images/grb061028_astrometry.gif

This Circular is an official product of the Swift XRT team.