Magnetized GRB outflow model: weak reverse shock emission and short energy transfer timescale

We show that the absence of the bright optical flashes in most {\it Swift} Gamma-ray Burst (GRB) afterglows can be explained, if the reverse shock region is magnetized with a $\sigma \sim 1$, or the emission spectrum of the electrons accelerated in the mildly magnetized ($0.1<\sigma<1$) reverse shock front is very soft, or the reverse shock of a non-magnetized outflow is sub-relativistic, where $\sigma$ is the ratio of the magnetic energy flux to the particle energy flux. We also find that for $\sigma\gg 1$, the energy transfer between the magnetized ejecta and the forward shock may be too quick to account for the shallow decline phase that is well detected in many {\it Swift} GRB X-ray afterglows.