2007MNRAS.374..525W

With the successful launch of the Swift Gamma-ray Burst Explorer, it is widely expected that the prompt optical flashes like GRB990123 would be easily detected. However, the observations show that for a number of Gamma-ray bursts (GRBs) no early optical flash has been detected, which indicates that the reverse shock emission must be suppressed. Here we explore the possibility that the optical flash may arise from the internal shock emission. For GRB990123 and GRB060111b, although their optical emission are not correlated with the gamma-ray emission, we propose here that their optical and gamma-ray emission may arise from different internal shocks (which can be formed by collision of different shells), and find that, under certain circumstances, the optical flashes of GRB990123 and GRB060111b can well be explained by the internal shock model. For GRB041219a, the prompt optical emission was correlated with the gamma-ray emission, which can also be explained by the internal shock model if we assume the optical emission was the low-energy extension of the gamma-ray emission, and we find its redshift is about z ∼ 0.2. As for GRB050904, we have shown in previous paper that the optical flash was produced by synchrotron radiation and the X-ray flare was produced by the synchrotron-self-Compton (SSC) mechanism. Therefore we conclude that the early optical flashes of GRBs can usually arise from the internal shock emission. Meanwhile in our model since the shells producing the optical flashes would be easily disrupted by other shells, so we suggest that the bright optical flash should not be common in GRBs. In addition, we also discussed the SSC emission in the internal shock model, and find that for different values of parameters, there would be several kinds of high-energy emission (at ∼100 keV, ∼10 MeV or GeV) accompanying the optical flash. For a burst like GRB990123, a GeV flare with fluence about 10^–8 erg/cm2/s is expected, which might be detected by the GLAST satellite.