A Unified Model of the Prompt Optical Emission of Gamma-Ray Bursts

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 the optical light curve correlates with those in the X-rays and/or gamma-rays or not, which will be tested in the near future.