2007ApJ...659L..95D

The observational diversity of optical emission, which coincides with prompt gamma-ray bursts (GRBs), has been discovered in the recent Swift era. We show that on the assumption of the synchrotron radiation for the observed energy range below the X-ray band, the observed diversity can be explained using the internal shock model by taking into account a high-latitude emission and the spectral change due to the synchrotron self-absorption. It may even be possible in our model to include bright optical flashes found, e.g., in GRB 990123. The prediction of our model is that the spectral index in the optical band is dependent on whether or not the optical light curve correlates with those in the X-rays and/or γ-rays, which will be tested in the near future.