Kinetic simulations of mildly relativistic shocks – I. Particle acceleration in high Mach number shocks
Kinetic simulations of mildly relativistic shocks – I. Particle acceleration in high Mach number shocks
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DOI：
10.1093/mnras/stz232
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发表时间：
2018-09
影响因子：
4.8
通讯作者：
P. Crumley;D. Caprioli;S. Markoff;A. Spitkovsky
中科院分区：
文献类型：
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作者：
P. Crumley;D. Caprioli;S. Markoff;A. Spitkovsky
我们使用空前大的横向盒尺寸的完全动力学粒子在单元内模拟来研究在速度≈0.75c和马赫数为15的弱磁化轻度相对论性激波中的粒子加速度。我们检查了亚腔（准平行）和超腔（准垂直）磁场取向。我们发现准平行激波是由由返回离子驱动的丝状非共振（Bell）不稳定性介导的，在与离子陀螺半径相当的尺度上产生磁波动。在准平行激波中，电子和离子都被加速成非热幂律，其最大能量随时间线性增长。电子的上游受热很小，两种物质进入激波前沿处于粗略的热平衡状态。减震器结构复杂；返回的非热离子的电流将上游的空腔排出，这些空腔一旦平流到下游就会形成被放大的磁场细丝。在晚些时候，10%的激波能量变成了非热质子，10%变成了磁场。我们发现，正确捕获由非热离子驱动的磁湍流对于正确测量非热电子εe的能量分数非常重要。我们在v = 0.75c的准平行激波中发现εe ~ 5 × 10−4，比在非相对论性激波模拟中测量到的值略大。在准垂直激波中，离子或电子中没有非热幂律。准平行激波中的离子加速效率表明，能够承载轻度相对论性准平行激波的天体物理对象——例如，活动星系核或微类星体的喷流——可能是宇宙射线及其次级射线（如伽马射线和中微子）的重要来源。
We use fully kinetic particle-in-cell simulations with unprecedentedly large transverse box sizes to study particle acceleration in weakly magnetized mildly relativistic shocks travelling at a velocity ≈ 0.75c and a Mach number of 15. We examine both subluminal (quasi-parallel) and superluminal (quasi-perpendicular) magnetic field orientations. We find that quasi-parallel shocks are mediated by a filamentary non-resonant (Bell) instability driven by returning ions, producing magnetic fluctuations on scales comparable to the ion gyroradius. In quasi-parallel shocks, both electrons and ions are accelerated into non-thermal power laws whose maximum energy grows linearly with time. The upstream heating of electrons is small, and the two species enter the shock front in rough thermal equilibrium. The shock’s structure is complex; the current of returning non-thermal ions evacuates cavities in the upstream that form filaments of amplified magnetic fields once advected downstream. At late times, 10 per cent of the shock’s energy goes into non-thermal protons and ${\gtrsim }10{{\ \rm per\ cent}}$ into magnetic fields. We find that properly capturing the magnetic turbulence driven by the non-thermal ions is important for properly measuring the energy fraction of non-thermal electrons, εe. We find εe ∼ 5 × 10−4 for quasi-parallel shocks with v = 0.75c, slightly larger than what was measured in simulations of non-relativistic shocks. In quasi-perpendicular shocks, no non-thermal power-law develops in ions or electrons. The ion acceleration efficiency in quasi-parallel shocks suggests that astrophysical objects that could host mildly relativistic quasi-parallel shocks – for example, the jets of active galactic nuclei or microquasars – may be important sources of cosmic rays and their secondaries, such as gamma-rays and neutrinos.