磁星激发的超新星爆炸和GRB喷流
Magnetar-energized supernova explosions and GRB-jets
摘要：
利用轴对称广义相对论MHD模拟方法，研究了偶极磁场为B=10^{15}G，自转周期为2 ms的磁星诞生后，核坍缩超新星爆发的早期演化过程.数值模型显示出高度准直的磁驱动射流非常早。射流是超Alfvenic，但保持亚快，直到模拟结束（t=0.2s）。喷流中释放的功率约为3 × 10 ^{50}erg/s，这意味着自旋下降时间约为37 s。磁星的总旋转能量E~10^{52}erg足以驱动超新星爆发，但尚不清楚这些能量中有多大一部分可以转移到恒星包层。考虑到观测到的喷流的传播速度v_p~0.17c，预计它们将在几秒钟内穿过祖先，在此之后，大部分释放的旋转能量将被这些喷流简单地带到周围的空间中。我们的研究结果提供了第一个或多或少的自洽的数值模型的中央引擎能够生产，在超新星设置和长期的基础上，准直喷流有足够的功率来解释长时间的伽玛暴和他们的余辉。虽然喷流的速度相对较低（vj ~0.5c$），但原中子星的冷却最终将导致其磁层的磁化强度大大提高，喷流的渐近速度也达到超相对论性。考虑到相对较长的冷却时间尺度，我们仍然期望喷流在爆发时只是弱相对论性的。这导致了一个模型的伽玛暴喷流与系统的纵向变化的洛伦兹因子，可能有特定的观测签名都在提示和余辉发射。
英文摘要：
In this paper we report on the early evolution of core-collapse supernova explosion following the birth of a magnetar with the dipolar magnetic field of B=10^{15}G and the rotational period of 2ms, which was studied by means of axisymmetric general relativistic MHD simulations. The numerical models exhibit highly collimated magnetically-driven jets very early on. The jets are super-Alfvenic but remain sub-fast until the end of the simulations (t=0.2s). The power released in the jets is about 3x10^{50}erg/s which implies the spin-down time of ~37s. The total rotational energy of the magnetar, E~10^{52}erg, is sufficient to drive hypernova but it is not clear as to how large a fraction of this energy can be transfered to the stellar envelope. Given the observed propagation speed of the jets, v_p~0.17c, they are expected to traverse the progenitor in few seconds and after this most of the released rotational energy would be simply carried away by these jets into the surrounding space. Our results provide the first more or less self-consistent numerical model of a central engine capable of producing, in the supernova setting and on a long-term basis, collimated jets with sufficient power to explain long duration GRBs and their afterglows. Although the flow speed of our jets is relatively low, v_j~0.5c$, the cooling of proto-neutron star will eventually result in much higher magnetization of its magnetospheres and ultra-relativistic asymptotic speeds of the jets. Given the relatively long cooling time-scale we still expect the jets to be only weakly relativistic by the time of break out. This leads to a model of GRB jets with systematic longitudinal variation of Lorentz factor which may have specific observational signatures both in the prompt and the afterglow emission.
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