Cosmic Ray Acceleration at Ultrarelativistic Shock Waves: Effects of a “Realistic” Magnetic Field Structure
Cosmic Ray Acceleration at Ultrarelativistic Shock Waves: Effects of a “Realistic” Magnetic Field Structure
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DOI：
10.1086/500541
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发表时间：
2005-10
期刊：
影响因子：
--
通讯作者：
J. Niemiec;M. Ostrowski
中科院分区：
文献类型：
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作者：
J. Niemiec;M. Ostrowski
用Monte Carlo方法研究了超相对论（γ ~ 5-30）激波的一阶费米加速过程。通过对激波附近湍流磁场中的精确粒子轨迹积分，导出了加速粒子谱。冲击上游的磁场模型假设在宽的波矢范围内的有限振幅扰动，并具有预定义的波功率谱，施加在与冲击法线成一定角度倾斜的平均场分量上。下游场结构作为压缩的上游场而获得。我们表明，在超光速冲击的主要加速过程是在冲击的粒子压缩。形成充满活力的光谱尾巴是可能的，在有限的能量范围内，只有高度扰动的磁场。在考虑的共振能量范围内，在低能量处发生光谱的截止。这些光谱特征的结果，从各向异性字符的粒子运输的磁场下游的冲击，场压缩产生有效的二维扰动。我们还提出了平行冲击的结果。由于激波处的湍流场压缩，对于较大的湍流振幅，加速过程变得无效，在这种情况下，恢复了在斜激波中观察到的特征。对于小振幅扰动，粒子谱形成在宽的能量范围内，和修改的加速过程中，由于长波扰动的存在，观察到，如先前报道的轻度相对论冲击。平行激波有效加速所需的临界湍流振幅随激波洛伦兹因子γ的增大而减小。在亚光速和超光速激波中，γ的增加会导致截止能量较低的陡峭光谱。在我们的模拟中假设的“现实”背景条件下获得的光谱不收敛到文献中声称的“通用”光谱指数。因此，一阶费米加速在天体物理源中的作用相对论冲击需要认真的重新分析。
First-order Fermi acceleration processes at ultrarelativistic (γ ~ 5-30) shock waves are studied with Monte Carlo simulations. The accelerated particle spectra are derived by integrating the exact particle trajectories in a turbulent magnetic field near the shock. The magnetic field model upstream of the shock assumes finite-amplitude perturbations within a wide wavevector range and with a predefined wave power spectrum, imposed on the mean field component inclined at some angle to the shock normal. The downstream field structure is obtained as the compressed upstream field. We show that the main acceleration process at superluminal shocks is the particle compression at the shock. Formation of energetic spectral tails is possible in a limited energy range only for highly perturbed magnetic fields. Cutoffs in the spectra occur at low energies within the resonance energy range considered. These spectral features result from the anisotropic character of particle transport in the magnetic field downstream of the shock, where field compression produces effectively two-dimensional perturbations. We also present results for parallel shocks. Because of the turbulent field compression at the shock, the acceleration process becomes inefficient for larger turbulence amplitudes, and features observed in oblique shocks are recovered in this case. For small-amplitude perturbations, particle spectra are formed in the wide energy range, and modifications of the acceleration process due to the existence of long-wave perturbations are observed, as reported previously for mildly relativistic shocks. The critical turbulence amplitude required for efficient acceleration at parallel shocks decreases with increasing shock Lorentz factor γ. In both subluminal and superluminal shocks, an increase of γ leads to steeper spectra with lower cutoff energies. The spectra obtained for the "realistic" background conditions assumed in our simulations do not converge to the "universal" spectral index claimed in the literature. Thus, the role of the first-order Fermi acceleration in astrophysical sources hosting relativistic shocks requires serious reanalysis.