GCN Circular 14056

D. A. Perley (Caltech), S. B. Cenko, A. N. Morgan (UC Berkeley), and T. 
Kruehler (DARK) report:

We observed the position of GRB 121209A with the Low Resolution Imaging 
Spectrograph (LRIS) on Keck I on the night of UT December 11.  We 
acquired two 900-second spectra of the GROND afterglow candidate 
(Kruehler et al., GCN 14049
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) at a mean UT of 05:15 (1.30 day after the 
burst) followed by single exposure each of U- and I-band imaging at a 
mean UT of 06:33 (1.36 day after the burst).  The observations were 
affected by clouds and poor seeing, especially the imaging epoch.

We detect a source at the position of the GROND object in the I-band 
imaging frame.  Performing photometry relative to SDSS stars, we measure 
a magnitude of

I = 23.2 +/- 0.2 mag  (I=23.6 AB)

Which is consistent with the magnitude at 2.4 hours after the burst 
reported by Kruehler et al.  The lack of fading over this time range 
suggests that the source is not a GRB afterglow.  However, it may be the 
event's host galaxy.   Given the bright X-ray afterglow concurrent with 
the lack of an optical/IR counterpart in PAIRITEL, Gemini GROND and 
RATIR observations (GCNs 14047
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, 14048
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, 14049
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, 14050
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), GRB 121209A is 
"dark" burst (beta_OX ~< 0, taking the GROND host magnitudes as upper 
limits).

Preliminary analysis of our 2D spectra shows no evidence of line 
emission across the spectral range (effectively continuous from the 
atmospheric cutoff to 10300 Angstroms), despite detection of a (weak) 
continuum trace down to approximately 10000 Angstroms.  If this is a 
star-forming galaxy, this absence of lines (in particular, from OII) 
would suggest a moderately high redshift (z>~1.7).  Deeper spectroscopy 
and in particular infrared observations would be needed to confirm this 
hypothesis.  If this is a distant galaxy, its apparent optical 
brightness indicates that it quite luminous.