arXivDaily arXiv每日学术速递 周一至周五更新
arXiv 0901.4776astro-ph.HE
收藏

单极磁层中磁-动能转换效率

Efficiency of Magnetic to Kinetic Energy Conversion in a Monopole Magnetosphere

Alexander Tchekhovskoy, Jonathan C. McKinney, Ramesh Narayan

摘要:

旋转磁化致密物体的无约束相对论性外流通常通过假设场的几何形状在大半径处近似为分裂的几何形状来很好地模拟。早期的工作已经表明,这种无约束流具有从磁能到动能的低效转换。这导致了这样的结论,即理想的磁流体动力学(MHD)过程无法解释观测到的,例如,蟹状星云的脉冲星在大半径的地方产生风,在那里能量转换显得很有效。此外,作为天体物理学喷流的模型,由于外部约束喷流似乎是更有效的喷流加速器,因此已经放弃了外磁场几何。我们进行时间相关的轴对称相对论MHD模拟,以找到稳定状态的解决方案,从一个紧凑的物体赋予一个磁场几何风。我们的模拟遵循流出10个数量级的距离紧凑的对象,这是足够大的研究磁化风的初始“加速区”以及渐近“滑行区”。“我们得到了令人惊讶的结果,即在极地地区,加速实际上是{ \it有效}的,尽管没有受到外部介质的限制,但仍会产生喷流。我们的模型包含有足够的能量来解释中等能量的长和短伽马射线暴(GRB)事件的喷流(~10^{51}--10 ^$> 52} erg),准直成窄开度角(开口半角θ_j \approx 0.03 rad),在大半径处变成物质主导(单位物质能流的电磁能流σ<1),并以超相对论性洛伦兹因子(基准模型的γ j ~ 200)运动。(节略)

英文摘要:

Unconfined relativistic outflows from rotating, magnetized compact objects are often well-modeled by assuming the field geometry is approximately a split-monopole at large radii. Earlier work has indicated that such an unconfined flow has an inefficient conversion of magnetic energy to kinetic energy. This has led to the conclusion that ideal magnetohydrodynamical (MHD) processes fail to explain observations of, e.g., the Crab pulsar wind at large radii where energy conversion appears efficient. In addition, as a model for astrophysical jets, the monopole field geometry has been abandoned in favor of externally confined jets since the latter appeared to be generically more efficient jet accelerators. We perform time-dependent axisymmetric relativistic MHD simulations in order to find steady state solutions for a wind from a compact object endowed with a monopole field geometry. Our simulations follow the outflow for 10 orders of magnitude in distance from the compact object, which is large enough to study both the initial "acceleration zone" of the magnetized wind as well as the asymptotic "coasting zone." We obtain the surprising result that acceleration is actually {\it efficient} in the polar region, which develops a jet despite not being confined by an external medium. Our models contain jets that have sufficient energy to account for moderately energetic long and short gamma-ray burst (GRB) events (~10^{51}--10^{52} erg), collimate into narrow opening angles (opening half-angle θ_j \approx 0.03 rad), become matter-dominated at large radii (electromagnetic energy flux per unit matter energy flux σ<1), and move at ultrarelativistic Lorentz factors (γ_j ~ 200 for our fiducial model). (abridged)

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

↑