GRB 990123:: reverse and internal shock flashes and late afterglow behaviour

The prompt (t less than or similar to 0.16d) light curve and initial 9th-magnitude optical flash from GRB 990123 can be attributed to a reverse external shock, or possibly to internal shocks. We discuss the time decay laws and spectral slopes expected under various dynamical regimes, and the constraints imposed on the model by the observations, arguing that they provide strongly suggestive evidence for features beyond those in the simple standard model. The longer term afterglow behaviour is discussed in the context of the forward shock, and it is argued that, if the steepening after 3 d is due to a jet geometry, this is likely to be a result of jet-edge effects, rather than sideways expansion.