2003ApJ...596..401N

We perform two-dimensional relativistic hydrodynamic simulations in the context of collapsar model. Calculations of explosive nucleosynthesis are also accomplished. We investigate the influence of the structure of the progenitor and energy deposition rate on the resulting explosive nucleosynthesis, assuming that ⁵⁶Ni is mainly synthesized in the jet launched by the neutrino heating. We show that the amount of ⁵⁶Ni is very sensitive to the energy deposition rate. Thus, we conclude that it is quite natural to detect no underlying supernova in some X-ray afterglows, such as GRB 010921. We also point out the possibility that the relative abundance of the elements with an intermediate mass number, such as Si and S, in the X-ray afterglow of GRB 011211 may be naturally explained if the energy deposition rate at the central engine is relatively long because little ⁵⁶Ni should be synthesized under such an environment. If this discussion is true, there should be a correlation between the line features in the X-ray afterglow and the duration of the gamma-ray burst. It should be noted that the duration of GRB 011211 is 270 s, making it the longest burst ever observed by BeppoSAX, although it suffers from the effect of redshift (z_host=2.14), and supporting our conclusion. Our results also suggest that the type I collapsar model, in which the energy deposition rate is relatively low (E{dot}∼10⁵¹ ergs/s), might have difficulty in reproducing the observed amount of ⁵⁶Ni in a hypernova such as SN 1998bw. This means that models of the mechanism of the central engine of a hypernova-accompanying gamma-ray burst may be constrained by the requirements of explosive nucleosynthesis.