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GCN Circular 12588

GRB 111117A: Refined Chandra Astrometry
2011-11-23T20:58:42Z (13 years ago)
Edo Berger at Harvard <>
E. Berger, W. Fong (Harvard), T. Sakamoto (GSFC/UMBC), and E. Troja
(NASA/GSFC/ORAU) report:

"We re-analyzed the Chandra observation of the short GRB 111117A (Sakamoto
et al. GCN 12580) for the purpose of clarifying and refining the astrometry
relative to the location of the host galaxy (using an optical image from
Magellan/IMACS:  Fong et al. GCN 12566).  Using 4 objects in common to the
Chandra data and the IMACS image (which is itself tied to USNO-B with 0.25"
rms) we find a correction to the native Chandra astrometry of:

dRA = -0.54+/-0.17"
dDEC = +0.34+/-0.14"

substantially improved relative to the 0.6" systematic uncertainty of the
native Chandra astrometry (Sakamoto et al. GCN 12580).  With this
correction, the position of the X-ray counterpart of GRB 111117A is (J2000):

RA = 00:50:46.257
DEC = +23:00:40.01

with a statistical uncertainty of about 0.13" in each coordinate.

Relative astrometry compared to the centroid of the host galaxy indicates a
projected angular offset of 1.0".  Although the redshift of the host galaxy
is not known at the present, this roughly corresponds to 6-8 kpc at
redshifts of z>0.5 (typical of short GRB offsets: Fong et al. 2010, ApJ,
708, 9)."
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