LONG-TERM EVOLUTION OF SLOWLY ROTATING COLLAPSAR IN SPECIAL RELATIVISTIC MAGNETOHYDRODYNAMICS

We present our numerical results of two-dimensional magnetohydrodynamic (MHD) simulations of the collapse of rotating massive stars in light of the collapsar model of gamma-ray bursts (GRBs). Pushed by recent evolution calculations of GRB progenitors, we focus on lower angular momentum of the central core than those taken mostly in previous studies. By performing special relativistic simulations including both realistic equation of state and neutrino cooling, we follow a long-term evolution of the slowly rotating collapsars up to similar to 10 s, accompanied by the formation of jets and accretion disks. Our results show that for the GRB progenitors to function as collapsars, there is a critical initial angular momentum, below which matter is quickly swallowed to the central objects, no accretion disks and no MHD outflows are formed. When larger than the criteria, we find the launch of the MHD jets in the following two ways. For models with stronger initial magnetic fields, the magnetic pressure amplified inside the accretion disk can drive the MHD outflows, which makes the strong magnetic explosions like a magnetic tower. For models with weaker initial magnetic fields, the magnetic tower stalls first and the subsequent MHD outflows are produced by the magnetic twisting of the turbulent inflows of the accreting material from the equatorial to the polar regions. Regardless of the difference in the formation, the jets can attain only mildly relativistic speeds with the explosion energy less than 10(49) erg. To obtain stronger neutrino energy depositions in the polar funnel regions heated from the accretion disk, we find that smaller initial angular momentum is favorable. This is because the gravitational compression makes the temperature of the disk higher. Due to high neutrino opacity inside the disk, we find that the luminosities of nu(e) and (nu) over bar (e) become almost comparable, which is advantageous for making the energy deposition rate larger. We discuss how the energy deposition can be as efficient as the magnetically driven processes for energetizing jets. Among the computed models, we suggest that the model with the initial angular momentum of j similar to 1.5j(lso) (j(lso): the angular momentum of the last stable orbit) and with initial magnetic field strength of similar to 10(10) G, provides a most plausible condition for making fireballs for GRBs, because such model is appropriate not only for producing the MHD outflows quickly by the magnetic towers, but also for obtaining the stronger neutrino heating in the evacuated polar funnel.