伽玛暴中的磁场和宇宙射线。自相似无碰撞前震
Magnetic fields and cosmic rays in GRBs. A self-similar collisionless foreshock
摘要：
被激波加速的宇宙射线在前震区域形成了一个向外射出的粒子流分布。如果与激波和宇宙射线能量学相比，环境场可以忽略不计，则通过流动（韦伯型）不稳定性，可以在激波上游产生更强的磁场。在这里，我们开发了一个自相似模型的前震区，并计算其结构，例如，磁场强度、其相干尺度等，与冲击波的距离有关我们的模型表明，整个前震区域的厚度$\sim R/(2Γ_{\rm sh}^2)$，在晚余辉阶段的冲击半径相比，$Γ_{\rm sh}\sim1$，可以填充大规模的和相当强的磁场（亚高斯强度与顺序$10^{17} {\rm cm}$的相干长度）相比，典型的星际介质磁场。在前震区存在这样的场对于在激波处实现高效率的费米加速是重要的。来自前震场中的加速电子的辐射可以构成在UV/光学通过无线电频带中辐射的单独的发射区域，这取决于时间和冲击参数。我们还推测，这些场最终被传输到激波下游，可以大大提高伽马射线暴余辉激波的辐射效率。
英文摘要：
Cosmic rays accelerated by a shock form a streaming distribution of outgoing particles in the foreshock region. If the ambient fields are negligible compared to the shock and cosmic ray energetics, a stronger magnetic field can be generated in the shock upstream via the streaming (Weibel-type) instability. Here we develop a self-similar model of the foreshock region and calculate its structure, e.g., the magnetic field strength, its coherence scale, etc., as a function of the distance from the shock. Our model indicates that the entire foreshock region of thickness $\sim R/(2Γ_{\rm sh}^2)$, being comparable to the shock radius in the late afterglow phase when $Γ_{\rm sh}\sim1$, can be populated with large-scale and rather strong magnetic fields (of sub-gauss strengths with the coherence length of order $10^{17} {\rm cm}$) compared to the typical interstellar medium magnetic fields. The presence of such fields in the foreshock region is important for high efficiency of Fermi acceleration at the shock. Radiation from accelerated electrons in the foreshock fields can constitute a separate emission region radiating in the UV/optical through radio band, depending on time and shock parameters. We also speculate that these fields being eventually transported into the shock downstream can greatly increase radiative efficiency of a gamma-ray burst afterglow shock.
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