TITLE: GCN GRB OBSERVATION REPORT NUMBER: 464 SUBJECT: GRB 991208 Keck Spectroscopy DATE: 99/12/17 02:52:34 GMT FROM: Josh Bloom at CIT GRB 991208 Keck Spectroscopy J. S. Bloom, A. Diercks, S. G. Djorgovski, S. R. Kulkarni (Caltech), A. V. Filippenko (UCB), on behalf of the Caltech-NRAO-CARA GRB Collaboration report: "On 15.638 December 1999 UT, A. Filippenko obtained a 900-s spectrum using the Low-Resolution Imaging Spectrometer (LRIS; Oke et al. 1995) on the Keck II 10-m telescope of the transient associated with GRB 991208 (GCN #450; GCN #451; GCN #452). The grating was 300 l/mm giving an effective wavelength coverage of 3850 Ang to 8850 Ang with a dispersion of ~2.47 Ang/pixel. The optical transient is better detected in this spectrum compared to the observations of December 14, 1999 and reported earlier (GCN #460). Reductions of the December 15 data reveal an emission line at lambda=8541 Ang (preliminary wavelength calibration). The presence of continuum emission blueward of this line effectively rules out a Ly-alpha origin. The two most likely candidate identifications of this emission line are [OII] 3727 Ang or H-alpha 6563 Ang. If the former, then the redshift implied, presumably that of the host galaxy, is z=1.29. Coupled with the bright fluence above 25 keV (10^-4 erg cm^-2; GCN #450) this implies an isotropic emission (restframe 30-2000 keV) of ~4 x 10^{53} erg (Omega=0.2, Lambda=0, H_0 = 65 km/s/Mpc). Such high energetics could be relaxed by collimated emission as suggested by the steep decay in the optical light curve. If instead we associate the emission line with H-alpha 6563 Ang, then the implied redshift is z=0.30, making it the lowest redshift of all cosmological GRBs. The associated isotropic energy loss would be ~2 x 10^{52} erg (Omega=0.2, Lambda=0, H_0 = 65 km/s/Mpc), consistent with the energies of GRB 970508, 970228, and GRB 980613. We disfavor this low-redshift hypothesis since the [OIII] 4959,5007 lines should be present in our spectrum but we have not detected these lines. Nevertheless, if the low redshift proves correct, then GRB 991208 would be an excellent candidate for a detectable supernova component in the the coming month." This message may be cited.