Decay of the GRB 990123 optical afterglow: Implications for the fireball model

Broad-band (ultraviolet to near-infrared) observations of the intense gamma ray burst GRB 990123 started similar to 8.5 hours after the event and cont inued until 18 February 1999. When combined with other data, in particular from the Robotic Telescope and Transient Source Experiment (ROTSE) and the Hubble Space Telescope (HST), evidence emerges for a smoothly declining lig ht curve, suggesting some color dependence that could be related to a cooli ng break passing the ultraviolet-optical band at about 1 day after the high -energy event. The steeper decline rate seen after 1.5 to 2 days may be evi dence for a collimated jet pointing toward the observer.