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GCN Circular 10339

GRB 100117A: Gemini South Imaging and CFHT Pre-Explosion Images
2010-01-18T02:06:23Z (14 years ago)
S. Bradley Cenko at Caltech <>
S. B. Cenko, D. A. Perley, J. S. Bloom, B. E. Cobb, and A. N. Morgan (UC
Berkeley) report on behalf of a larger collaboration:

We have obtained pre-explosion images of the field of GRB100117A from the
Canada-France-Hawaii MegaCam archive.  Images were obtained in the g', r',
and z' filters on 26 December 2005, and in the i' filter on 12 November
2007.  Exposure times ranged from 240 s in g'-band to 500 s in i'-band.

Inside the candidate XRT afterglow localization (de Pasquale; GCN 10336),
we find four sources, with the following positions and R- and I-band
magnitudes (calibrated relative to several USNO-B1 sources in the field;
expected systematic uncertainty ~ 0.3 mag):

Source 	RA		Dec (J2000.0)		R	I
S1	00:45:04.82 	-01:35:47.7 		23.1	21.0
S2	00:45:04.78 	-01:35:45.5 		23.6	22.9
S3	00:45:04.70 	-01:35:42.0 		24.5	22.9
S4	00:45:04.53 	-01:35:49.2 		> 24.7	23.6

We have also obtained a single epoch of acquisition imaging in i'-band of
the field with GMOS on Gemini South beginning at 00:46 on 18 January 2010
(~ 4.7 hours after the burst).  We detect only S1 in this image, at a
magnitude consistent with the value derived from the CFHT image.

Furthermore, given the uncertainty in our astrometry (~ 0.5" in each
coordinate), we believe it likely this is the same source identified by
Xu et al. (GCN 10337) at R = 22.5 +/- 0.5.  We therefore have no evidence
that this source has varied.

Further observations are planned.

We wish to thank the Gemini Observatory staff, in particular Peter Pessev,
for assistance with these observations.
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