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GCN Circular 251

Subject
GRB 990123: Updated Keck Spectroscopy Results
Date
1999-02-05T02:31:18Z (26 years ago)
From
George Djorgovski at Caltech/Palomar <george@oracle.caltech.edu>
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 
collaboration:

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 249Not found
 
 
).

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 243Not found
 
 
) and in the 
R band (Yadigaroglu and Halpern, GCN 248Not found
 
 
) 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 219Not found
 
 
).

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 249Not found
 
 
).  No absorption or emission lines corresponding to this 
redshift are seen in the spectrum of the OT.  

This report may be cited.