TITLE: GCN GRB OBSERVATION REPORT NUMBER: 206 SUBJECT: GRB 990123 Optical Follow-up DATE: 99/01/24 00:01:51 GMT FROM: Josh Bloom at CIT GRB 990123 Optical Follow-Up J. S. Bloom, R. R. Gal, L. L. Lubin, J. Mulchaey, S. C. Odewahn (CIT), S. R. Kulkarni report on behalf of the Caltech-NRAO-CARA GRB Collaboration: "We obtained a single B-band image (300-s) at the Palomar 200-inch of the field of GRB 990123 localized by BeppoSAX (GCN #200). Using images obtained of the Landolt standard field 98, we derived a photometric zeropoint for the GRB data. The transient first reported in Odewahn, Bloom, Kulkarni (GCN #202) is well-detected at B = 18.93 +/- 0.03 mag (Jan 23.578 UT). The uncertainty includes the systematic and statistical uncertainties though no color term was applied. For reference, we find the following B = 19.59 +/- 0.04 for the object at position ra: 15:25:32.7, dec: +44:44:29.7 (J2000). Absolute astrometry was obtained by comparison of 34 objects near the optical transient with the USNO-A2.0 Catalogue. The r.m.s. uncertainties of the astrometry are 0.28 arcsec (ra) and 0.26 arcsec (dec). We find the position of the optical transient to be, ra: 15:25:30.34, dec: +44:45:59.1 (J2000). Based on the POSS-II F-Plate of this field, the purported host galaxy (see GCN #202) appears offset from the OT by 1.8 arcsec (+/- 0.4 arcsec) to the South. Further analysis is underway. This message may be cited."