GRB 990123: Updated Keck Spectroscopy Results
S. G. Djorgovski, S. R. Kulkarni (CIT), G. D. Illingworth (UCSC),
D. D. Kelson (DTM), J. S. Bloom, S. C. Odewahn, R. R. Gal (CIT),
M. Franx (Leiden), P. van Dokkum (Groningen), D. Magee (UCSC), and
D. A. Frail (NRAO) note on behalf of the Caltech-UC-CARA-NRAO
Our re-reduction of the Keck spectrum of the optical transient associated
with GRB 990123 (Kelson et al., IAUC 7096) gives the following results:
We detect 12 (13) absorption lines in the spectrum of the OT, as follows:
W_obs,air W_rest,vac z Line ID
4843.74 1862.78 1.6010 Al III
5267.29 2026.14 1.6004 Zn II
5361.77 2062.23 1.6007 Cr II | blend
5361.77 2062.66 1.6002 Zn II |
5877.17 2260.78 1.6003 Fe II
6096.14 2344.21 1.6012 Fe II
6173.87 2373.73 1.6016 Fe II
6195.29 2382.76 1.6008 Fe II
6725.75 2586.64 1.6009 Fe II
6759.94 2600.18 1.6005 Fe II
7269.47 2796.35 1.6003 Mg II
7289.49 2803.53 1.6008 Mg II
7416.97 2852.97 1.6005 Mg I
The mean redshift is 1.6004 +- 0.0005 (random) +- 0.0005 (systematic).
This agrees to within the quoted error with the new determination of
the absorber redshift by Hjorth et al. (GCN 249).
We note the remarkably small velocity dispersion implied by these data,
less than about 60 km/s in the restframe, suggesting that the lines arise
from a single subgalactic-size cloud (which of course may be a part of the
host galaxy's ISM), rather than from an ansamble of clouds moving within
the potential well of a normal, massive galaxy. It is also possible that
the GRB host is a dwarf galaxy, in which case the object detected near the
line of sight both in the K band (Djorgovski et al., GCN 243) and in the
R band (Yadigaroglu and Halpern, GCN 248) may be a foreground galaxy.
No other convincing absorption systems, and no emission lines are detected
in these data, in the useful wavelength range of approximately 4700 to 9000
Angstroms. We do not detect Ca II H+K absorption, nor any other common
absorption lines, e.g., Na D, nor any common emission lines (e.g., [O II]
3727, H alpha, H beta, etc.) from either of the two absorption systems
originally proposed by Hjorth et al. (GCN 219).
We have also measured the redshift of the galaxy approximately 10 arcsec
west of the OT. From 4 relatively "clean" lines, Ca II H+K, H beta, and
H alpha, we derive for its redshift z = 0.2783 +- 0.0005. From 4 blended
lines, CH G-band 4300, Mg I 5173+5184, Fe I + Ca I 5267, and Na D 5893,
we derive z = 0.278 +- 0.001, again in an excellent agreement with Hjorth
et al. (GCN 249). No absorption or emission lines corresponding to this
redshift are seen in the spectrum of the OT.
This report may be cited.