TITLE: GCN GRB OBSERVATION REPORT NUMBER: 208 SUBJECT: GRB 990123: Optical Decay Slope DATE: 99/01/24 09:12:10 GMT FROM: Josh Bloom at CIT GRB 990123: Optical Decay Slope J. S. Bloom, S. R. Kulkarni, S. G. Djorgovski, S. C. Odewahn, R. R. Gal, L. M. Lubin (CIT), and D. A. Frail (NRAO) note on behalf of the Caltech-CARA-NRAO GRB collaboration: "After an initial rise to about 1 Jy, the early-time optical light-curve of the transient afterglow of GRB 990123 (as reported by the ROTSE Team; GCN #205) displayed a power-law decay. We note here that the decay appears to be well-continued by the later time (~3 hours) optical fluxes of the transient discovered at Palomar (GCN #201; GCN #206; GCN #207). The decay constant is approximately alpha = -1.5 (defined as F_nu (t) = const * t^alpha). This is consistent with the decay constants measured at late-times in other GRB afterglows. This suggests a strong phenomenological connection between the emission mechanism(s) at very early times (from tens of seconds to few minutes after the burst) and at later times (>~ few hours). The continuity of the power-law light curve represents a strong constraint for theoretical models of optical emission from GRBs and their afterglows. A lightcurve, which will be actively updated, may be obtained at http://astro.caltech.edu/~jsb/GRB/grb990123.html This message may be cited."