Precursors and Main Bursts of Gamma-Ray Bursts in a Hypernova Scenario

We investigate a "hypernova" model for gamma-ray bursts (GRBs), i.e., a massive C+O star model with relativistic jets. In this model, nonthermal precursors can be produced by the "first" relativistic shell ejected from the star. Main GRBs are produced behind the first shell by the collisions of several relativistic shells. They become visible to distant observers after the colliding region becomes optically thin. We examine six selected conditions using relativistic hydrodynamical simulations and simple analyses. Interestingly, our simulations show that subrelativistic (v ~0.8c) jets from the central engine are sufficient to produce highly relativistic ((gamma) > 100) shells. We find that the relativistic shells from such a star can reproduce observed GRBs with certain conditions. Two conditions are especially important. One is the sufficiently long duration of the central engine, >~100 s. The other is the existence of a dense shell somewhere behind the first shell. Under these conditions, both the existence and nonexistence of precursors, and the long delay between precursors and main GRBs, can be explained.