超相对论伽马射线暴外激波前体中加速离子流产生的激波涡度
Shock Vorticity Generation from Accelerated Ion Streaming in the Precursor of Ultrarelativistic Gamma-Ray Burst External Shocks
摘要：
我们研究了在超相对论冲击波中加速的非热离子（质子和原子核）与冲击波传播介质中预先存在的磁场的相互作用。虽然诸如扩散冲击加速的粒子加速过程可以加速离子和电子，但是被加速的电子遭受更大的辐射损失。在某些条件下，离子可以获得更高的能量，并且比电子更早到达激波，因此非热粒子可以部分电荷分离。为了补偿电荷分离，上游等离子体产生返回电流，当其流过磁场时，该返回电流驱动上游等离子体的横向加速和上游激波中密度对比的增长。如果流体受到冲击时密度对比很强，则在冲击过渡时会产生涡量。由此产生的湍流可以将冲击后的磁场放大到从伽马射线暴余辉光谱和光变曲线推断的水平。因此，由于上游不均匀性是由在冲击波中加速的离子引起的，因此即使冲击波传播到均匀密度的介质中，它们也是通用的。我们推测的全球结构的冲击前兆，并描绘了几个不同的物理制度，这些制度是由一个不断增加的距离，从冲击，相应地，一个不断减少的密度，达到该距离的非热粒子分类。
英文摘要：
We investigate the interaction of nonthermal ions (protons and nuclei) accelerated in an ultrarelativistic blastwave with the pre-existing magnetic field of the medium into which the blastwave propagates. While particle acceleration processes such as diffusive shock acceleration can accelerate ions and electrons, the accelerated electrons suffer larger radiative losses. Under certain conditions, the ions can attain higher energies and reach farther ahead of the shock than the electrons, and so the nonthermal particles can be partially charge-separated. To compensate for the charge separation, the upstream plasma develops a return current, which, as it flows across the magnetic field, drives transverse acceleration of the upstream plasma and a growth of density contrast in the shock upstream. If the density contrast is strong by the time the fluid is shocked, vorticity is generated at the shock transition. The resulting turbulence can amplify the post-shock magnetic field to the levels inferred from gamma-ray burst afterglow spectra and light curves. Therefore, since the upstream inhomogeneities are induced by the ions accelerated in the shock, they are generic even if the blastwave propagates into a medium of uniform density. We speculate about the global structure of the shock precursor, and delineate several distinct physical regimes that are classified by an increasing distance from the shock and, correspondingly, a decreasing density of nonthermal particles that reach that distance.
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