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GCN Circular 256

Subject
GRB 990123: Discovery of the Probable Host Galaxy
Date
1999-02-09T21:41:21Z (25 years ago)
From
George Djorgovski at Caltech/Palomar <george@oracle.caltech.edu>
GRB 990123:  Discovery of the Probable Host Galaxy

S. G. Djorgovski, S. R. Kulkarni, J. S. Bloom, G. Neugebauer, C. Koresko
(Caltech), L. Armus (IPAC), S. C. Odewahn, B. R. Oppenheimer, R. R. Gal 
(Caltech), N. Kobayashi (NAOJ), and D. A. Frail (NRAO), report on behalf 
of the Caltech-CARA-NRAO GRB collaboration: 

We confirm the detection of a faint galaxy approximately 0.6 arcsec due north 
from the optical transient (OT) associated with GRB 990123, in the K-band
images obtained with the NIRC instrument at the Keck-I 10-m telescope, on the
nights of 29 January 1999 and 6, 7, and 8 February 1999 UT.  The presence of
this object was already suggested in the Keck K-band images obtained on 27
January 1999 UT by Malkan et al., and its K-band magnitude was estimated to 
be about 22 to 23 (see Djorgovski et al., GCN 243).  The galaxy is clearly 
resolved from the OT in the images obtained on 29 January by Neugebauer and 
Armus; it has about an equal magnitude as the OT in the images taken on 7 
and 8 February by Kulkarni and Oppenheimer, i.e., K =~ 22 +- 0.7 mag.

The K-band light curve containing the light from both objects begins to show
a flattening due to the presence of this galaxy.  The deviation from the
power-law light curve in the K-band (Bloom et al., GCN 240), assumed to 
have the slope alpha = -1.15 (as measured in the r band), implies the galaxy 
magnitude K =~ 22.4 (+0.9, -0.4; 1-sigma), in a good agreement with the 
previous estimates.

We interpret this object as the most likely counterpart of the absorber
at z = 1.600 (IAUC 7096, GCN 219, GCN 249, GCN 251), and the probable
host galaxy of the GRB.  Its observed K-band magnitude is reasonable for
a normal galaxy at z = 1.6.

Analysis of the HST images of the field shows the same object (GCN 255),
and further details will be reported shortly.

A Keck image of the field will be posted at:
http://astro.caltech.edu/~george/grb/grb990123.html

This report is citeable.
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