TITLE: GCN GRB OBSERVATION REPORT NUMBER: 256 SUBJECT: GRB 990123: Discovery of the Probable Host Galaxy DATE: 99/02/09 21:41:21 GMT FROM: George Djorgovski at Caltech/Palomar 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.