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GCN Circular 5983

Subject
GRB 061210: Swift/XRT Astrometry Correction
Date
2006-12-26T14:57:29Z (17 years ago)
From
Judith Racusin at PSU <racusin@astro.psu.edu>
J. L. Racusin (PSU), O. Godet, (U. Leicester), J. K. Cannizzo (NASA/UMBC), 
D. Fox (PSU), and N. Gehrels (NASA/GSFC) report on behalf of the Swift XRT 
team:

We have re-analyzed the full XRT data set of GRB 061210.  XRT observed the 
field of GRB 061210 for a total exposure time of 73 ks between December 12 
and December 20, 2006 in Photon Counting mode.

Godet et al. (GCN 5921) identified 2 potential X-ray afterglow candidates 
(S1 & S2).  After further observations to characterize fading behavior, we 
have concluded that S2 is the X-ray afterglow.

To further improve the accuracy of the position previously quoted, we 
performed an astrometric correction using 66 ks of the total exposure time 
(when the satellite position was stable).  In this data set we find 22 
serendipitous X-ray sources detected with the XIMAGE detect algorithm with 
S/N > 3, 6 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 (Godet et al., GCN 5921) Source 2, of:

RA offset: +0.036s +/- 0.055s   Dec offset: +0.13 +/- 0.8"

and a new XRT astrometry corrected position of:

RA(J2000): 09 38 05.24
Dec(J2000): +15 37 16.5

with an estimated uncertainty of 3.4 arcseconds (radius, 90% containment).

This position is 1.1 arcseconds from the original refined XRT position of 
S2 given in Godet et al. (GCN 5921), and 2.6 arcseconds from the potential 
S2-G2 host galaxy described by Berger et al. (GCN 5922) and Mirabal et al. 
(GCN 5925) with z=0.41 (Cenko et al., GCN 5946).

A figure comparing all of these positions is available at: 
http://www.swift.psu.edu/images/grb061210_astrometry.gif

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