{
  "bibcode": "2012GCN.14056....1P",
  "body": "D. A. Perley (Caltech), S. B. Cenko, A. N. Morgan (UC Berkeley), and T. \nKruehler (DARK) report:\n\nWe observed the position of GRB 121209A with the Low Resolution Imaging \nSpectrograph (LRIS) on Keck I on the night of UT December 11.  We \nacquired two 900-second spectra of the GROND afterglow candidate \n(Kruehler et al., GCN 14049) at a mean UT of 05:15 (1.30 day after the \nburst) followed by single exposure each of U- and I-band imaging at a \nmean UT of 06:33 (1.36 day after the burst).  The observations were \naffected by clouds and poor seeing, especially the imaging epoch.\n\nWe detect a source at the position of the GROND object in the I-band \nimaging frame.  Performing photometry relative to SDSS stars, we measure \na magnitude of\n\nI = 23.2 +/- 0.2 mag  (I=23.6 AB)\n\nWhich is consistent with the magnitude at 2.4 hours after the burst \nreported by Kruehler et al.  The lack of fading over this time range \nsuggests that the source is not a GRB afterglow.  However, it may be the \nevent's host galaxy.   Given the bright X-ray afterglow concurrent with \nthe lack of an optical/IR counterpart in PAIRITEL, Gemini GROND and \nRATIR observations (GCNs 14047, 14048, 14049, 14050), GRB 121209A is \n\"dark\" burst (beta_OX ~< 0, taking the GROND host magnitudes as upper \nlimits).\n\nPreliminary analysis of our 2D spectra shows no evidence of line \nemission across the spectral range (effectively continuous from the \natmospheric cutoff to 10300 Angstroms), despite detection of a (weak) \ncontinuum trace down to approximately 10000 Angstroms.  If this is a \nstar-forming galaxy, this absence of lines (in particular, from OII) \nwould suggest a moderately high redshift (z>~1.7).  Deeper spectroscopy \nand in particular infrared observations would be needed to confirm this \nhypothesis.  If this is a distant galaxy, its apparent optical \nbrightness indicates that it quite luminous.",
  "circularId": 14056,
  "createdOn": 1355224656000,
  "email": "dperley@astro.caltech.edu",
  "subject": "GRB 121209A: Keck observations",
  "submitter": "Daniel Perley at Caltech  <dperley@astro.caltech.edu>",
  "eventId": "GRB 121209A"
}