GCN Circular 464
Subject
GRB 991208 Keck Spectroscopy
Event
Date
1999-12-17T02:52:34Z (26 years ago)
From
Josh Bloom at CIT <jsb@astro.caltech.edu>
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."
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