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GCN Circular 219

GRB 990123 Spectroscopic Redshifts
1999-01-25T11:22:11Z (25 years ago)
Jens Hjorth at U.Copenhagen <>
GRB 990123 Spectroscopic Redshifts

J. Hjorth (Copenhagen), M. I. Andersen (NOT), 
L. M. Cairos, N. Caon, M. Zapatero Osorio (IAC), 
H. Pedersen, B. Lindgren (Copenhagen), 
A. J. Castro Tirado (LAEFF, IAA), and E. Perez (IAA) report:

"Three 40 min spectra of GRB 990123 were obtained at NOT on Jan 24.24 UT 1999. 
Based on a preliminary reduction and analysis we detect the following 
ultraviolet absorption lines: Si II (152.6 nm), C IV (154.9 nm), Fe II 
(160.8 nm), Al II (167.1 nm), Zn II (202.6 and 206.3 nm), Fe II (234.4, 237.5, 
and 238.3 nm), and Fe II (258.7 and 260.0 nm) at a redshift of 1.600 +- 0.001. 
This value is in agreement with the redshift found by Kelson et al. (IAU Circ. 
7096).  Several additional absorption lines and two emission lines are detected.
There is one definite system at z = 0.286 +- 0.001 based on [OII] (372.7 nm) 
emission and Ca H and K (393.3 nm and 396.9 nm) absorption and another possible
system at z = 0.210 +- 0.002 based on [OII] emission and Ca K absorption.  One
of these systems are likely to be associated with the galaxy located 
1.8 +- 0.4" from the optical transient (Bloom et al., GCN Circ. 206).  This 
galaxy has an estimated redshift of z ~ 0.2 - 0.3 (Gal et al., GCN Circ. 213). 

These observations support the idea that GRB 990123 may be a gravitationally 
lensed gamma-ray burst (Djorgovski et al. GCN Circ. 216).  The 'source' 
(optical transient) redshift (z >= 1.60) and 'lens' (galaxy) redshift(s) 
(z = 0.29, z = 0.21) as well as the angular separation between the image and 
the lens are typical of those found among multiply imaged QSOs.  The existence
of several galaxies along the line of sight to the GRB enhances the lensing
cross section. If GRB 990123 is multiply lensed, a fainter GRB is expected to 
appear 2-3" north of GRB 990123, within a time scale of months.  In addition, 
two more GRBs may appear within days or weeks from now if GRB 990123 is 
quadruply lensed.  Continued monitoring is urged at all wavelengths in order to 
determine precise time delay(s).

The results reported in this circular are preliminary.  A more detailed
analysis is ongoing." 

This report may be cited.

[GCN OP NOTE:  This archived copy of Circular 219 has been reformatted
by the author, i.e. different than the copy actually distributed.]
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