Numerical Study on GRB-Jet Formation in Collapsars-猫眼课题宝

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2006-08

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S. Nagataki;Yukawa Inst. Kyoto Kyoto U.-Yukawa-Inst.-Kyoto-Kyoto-U.-1451901027;Menlo Park Kipac;R. Takahashi;A. Mizuta;M. P. I. Garching;T. Takiwaki

利用ZEUS-2D程序对产生伽玛暴喷流的崩塌星进行了二维磁流体力学模拟，考虑了实际状态方程、中微子冷却和加热过程、磁场以及来自中心黑洞和自引力的引力。在这项研究中发现，中微子加热过程并不是那么有效地发射喷气式飞机。研究还发现，喷流主要是由被缠绕效应放大的B{sub{Phi}}场发射的。然而，由于喷流中总能量与其余质量能量的比值不是几百倍，我们得出结论，本研究中所看到的喷流不是伽玛暴喷流。这一结果表明，广义相对论效应对产生伽玛暴喷流将是重要的，但这项研究没有包括这些效应。此外，磁场吸积盘仍可能在发射伽玛射线暴喷流中发挥重要作用，尽管需要对更长的物理时间进行模拟才能证实这一效应。结果表明，在吸积盘中合成了相当数量的{sup 56}Ni。因此，吸积盘更有可能提供足够的{sup 56}Ni来解释超新星的光度。结果还表明，由于每个重子具有高熵的电子俘获而产生的富中子物质沿极轴被抛射。此外，通过在该区域俘获{nu}{sube}，发现电子分数在黑洞附近的极轴附近变得大于0.5。因此，有可能在这些区域发生r-过程和r/p-过程核合成。最后，许多中子将从喷流中喷射出来，这意味着来自中子衰变的信号可能被观察到为余辉或伽马射线的光曲线的延迟凸起。

Two-dimensional magnetohydrodynamic simulations are performed using the ZEUS-2D code to investigate the dynamics of a collapsar that generates a GRB jet, taking account of realistic equation of state, neutrino cooling and heating processes, magnetic fields, and gravitational force from the central black hole and self gravity. It is found that neutrino heating processes are not so efficient to launch a jet in this study. It is also found that a jet is launched mainly by B{sub {phi}} fields that are amplified by the winding-up effect. However, since the ratio of total energy relative to the rest mass energy in the jet is not so high as several hundred, we conclude that the jets seen in this study are not be a GRB jet. This result suggests that general relativistic effects, which are not included in this study, will be important to generate a GRB jet. Also, the accretion disk with magnetic fields may still play an important role to launch a GRB jet, although a simulation for much longer physical time {approx} 10-100 s is required to confirm this effect. It is shown that considerable amount of {sup 56}Ni is synthesized in the accretion disk. Thus there will be amore » possibility for the accretion disk to supply sufficient amount of {sup 56}Ni required to explain the luminosity of a hypernova. Also, it is shown that neutron-rich matter due to electron captures with high entropy per baryon is ejected along the polar axis. Moreover, it is found that the electron fraction becomes larger than 0.5 around the polar axis near the black hole by {nu}{sub e} capture at the region. Thus there will be a possibility that r-process and r/p-process nucleosynthesis occur at these regions. Finally, much neutrons will be ejected from the jet, which suggests that signals from the neutron decays may be observed as the delayed bump of the light curve of the afterglow or gamma-rays.« less

Numerical Study on GRB-Jet Formation in Collapsars

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