Gamma-Ray Burst Physics

I have developed two-dimensional general relativistic magnetohydrodynamic (GRMHD) code. I have performed numerical simulations of collapsars using these codes and realistic progenitor models. In the GRMHD simulation, it is shown that a jet is launched from the center of the progenitor. We also performed simulations of collapsars with different Kerr parameters a = 0, 0.5, 0.9, 0.95. It is shown that a more rapidly rotating black hole is driving a more energetic jet. No jet is seen for the case of Schwartzschild black hole case, while the total energy of the jet is as large as 1050 erg for a rapidly rotating Kerr black hole case (a = 0.95). In order to explain the high luminosity of a GRB, it is concluded that a rapidly rotating black hole is favored ('faster is better'). We also performed two-dimensional hydrodynamic simulations in the context of collapsar model to investigate the explosive nucleosynthesis happened there. It is found that the amount of 56Ni is very sensitive to the energy deposition rate. This result means that the amount of synthesized 56Ni can be little even if the total explosion energy is as large as 1052 erg. Thus, some GRBs can associate with faint supernovae. Thus we consider it is quite natural to detect no underlying supernova in some X-ray afterglows.