TITLE:   GCN CIRCULAR
NUMBER:  5774
SUBJECT: GRB 061004: Swift/XRT Astrometry Correction
DATE:    06/10/31 23:38:37 GMT
FROM:    Judith Racusin at PSU  <racusin@astro.psu.edu>

J. L. Racusin (PSU), D. N. Burrows (PSU), K. L. Page (U Leicester), H. 
Ziaeepour (UCL-MSSL), and N. Gehrels (NASA/GSFC) report on behalf of the 
Swift XRT team:

We have re-analyzed the full XRT data set of GRB 061004.  XRT observed the 
field of GRB 061004 for a total exposure time of 61 ks between October 4 
and October 10, 2006 in Photon Counting mode.

Applying the new XRT boresight definition (Burrows et al., GCN 5750), we 
find a boresight corrected position of RA(J2000): 06h 31m 10.68s, 
Dec(J2000): -45d 54' 22.7", with an estimated uncertainty of 3.6 
arcseconds (radius, 90% containment).

To further improve the accuracy of this position, we performed an 
astrometry correction using 54 ks of the total exposure time (when the 
satellite position was stable).  In this data set we find 26 serendipitous 
X-ray sources detected with the XIMAGE detect algorithm with S/N > 3, 5 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 (Page et al., GCN 5695), of:

RA offset: +0.49s +/- 0.07s   Dec offset: +1.3"  +/- 1.0"

and a new XRT astrometry corrected position of:

RA(J2000):   06h 31m 10.94s
Dec(J2000): -45d 54' 23.3"

with an estimated uncertainty of 2.0 arcseconds (radius, 90% containment).

This position is 5.0 arcseconds from the original refined XRT position 
given in Page et al. (GCN 5695), 2.8 arcseconds from the boresight 
corrected XRT position, and 5.9 arcseconds from the faint VLT optical 
object described in Jakobsson et al. (GCN 5698).

A figure comparing all of these positions is available at: 
http://www.swift.psu.edu/images/grb061004_astrometry.gif

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