宇宙线在被超新星残余冲击波扫过的多相星际介质中的扩散
Diffusion of Cosmic Rays in a Multiphase Interstellar Medium Swept-Up by a Supernova Remnant Blast Wave
摘要：
超新星遗迹（SNRs）是最具能量的天体物理事件之一，被认为是银河系宇宙射线（CR）的主要来源。费米卫星最近的一份观测报告显示，信噪比的伽马射线谱中存在π介子衰变的特征。这提供了强有力的证据表明，高能质子在SNR中被加速。π介子衰变的实际γ射线辐射应取决于CR在星际介质中的扩散。为了定量分析从加速站点的高能量CR的扩散，我们已经进行了CR质子的测试粒子数值模拟使用三维磁流体动力学（MHD）模拟的星际介质被冲击波扫起。我们分析了在我们模拟的信噪比中几个pc量级的CRs的扩散，发现CRs的扩散精确地由Bohm扩散描述，这是有效加速所需的，至少对于实际的星际介质，能量超过30 TeV的粒子。虽然我们在模拟中发现了超扩散过程（传播距离与t^0.75成正比）的可能性，但在SNR中湍流的长度尺度上，它对CR扩散的影响是有限的。
英文摘要：
Supernova remnants (SNRs) are one of the most energetic astrophysical events and are thought to be the dominant source of Galactic cosmic rays (CRs). A recent report on observations from the Fermi satellite has shown a signature of pion decay in the gamma-ray spectra of SNRs. This provides strong evidence that high-energy protons are accelerated in SNRs. The actual gamma-ray emission from pion decay should depend on the diffusion of CRs in the interstellar medium. In order to quantitatively analyse the diffusion of high-energy CRs from acceleration sites, we have performed test particle numerical simulations of CR protons using a three-dimensional magnetohydrodynamics (MHD) simulation of an interstellar medium swept-up by a blast wave. We analyse the diffusion of CRs at a length scale of order a few pc in our simulated SNR, and find the diffusion of CRs is precisely described by a Bohm diffusion, which is required for efficient acceleration at least for particles with energies above 30 TeV for a realistic interstellar medium. Although we find the possibility of a superdiffusive process (travel distance proportional to t^0.75) in our simulations, its effect on CR diffusion at the length scale of the turbulence in the SNR is limited.
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