TITLE: GCN CIRCULAR NUMBER: 14056 SUBJECT: GRB 121209A: Keck observations DATE: 12/12/11 11:17:36 GMT FROM: Daniel Perley at Caltech 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) 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, 14048, 14049, 14050), 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.