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arXiv 1407.6728astro-ph.HE
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超新星遗迹中垂直激波的Cosmic ray加速度

Cosmic ray acceleration at perpendicular shocks in supernova remnants

Gilles Ferrand, Rebecca J. Danos, Andreas Shalchi, Samar Safi-Harb, Paul Edmon, Peter Mendygral

摘要:

超新星遗迹(SNRs)被认为通过扩散激波加速(DSA)机制将粒子加速到高能量。除了直接的等离子体模拟,所有的建模工作必须依赖于一个给定的形式的扩散系数,一个关键的参数,体现了高能带电粒子与磁湍流的相互作用。通常采用所谓的玻姆极限。在本文中,我们重新讨论的问题加速在垂直冲击,通过采用一个现实的模型垂直扩散。我们的系数在低动量(指数为$α$)时降为幂律动量,但在高动量(在某些特征动量$p_{\rm c}$以上达到一个常数值$κ_{\infty}$)时变得与粒子动量无关。我们首先提供了简单的分析表达式的最大动量,可以达到在给定的时间与这个系数。然后,我们进行随时间变化的数值模拟,以调查的颗粒分布的形状,可以得到的颗粒压力时,反作用于流。我们观察到,对于一个给定的指数$α$和注入水平,冲击修改是相似的$p_{\rm c}$的不同可能的值,而粒子谱显着不同。特别令人感兴趣的是,低的$p_{\rm c}$值往往会消除曾经被认为是典型的非线性DSA的反射,并导致陡峭的光谱,正如最近对银河系SNR的高能观测所要求的那样。

英文摘要:

Supernova remnants (SNRs) are believed to accelerate particles up to high energies through the mechanism of diffusive shock acceleration (DSA). Except for direct plasma simulations, all modeling efforts must rely on a given form of the diffusion coefficient, a key parameter that embodies the interactions of energetic charged particles with the magnetic turbulence. The so-called Bohm limit is commonly employed. In this paper we revisit the question of acceleration at perpendicular shocks, by employing a realistic model of perpendicular diffusion. Our coefficient reduces to a power-law in momentum for low momenta (of index $α$), but becomes independent of the particle momentum at high momenta (reaching a constant value $κ_{\infty}$ above some characteristic momentum $p_{\rm c}$). We first provide simple analytical expressions of the maximum momentum that can be reached at a given time with this coefficient. Then we perform time-dependent numerical simulations to investigate the shape of the particle distribution that can be obtained when the particle pressure back-reacts on the flow. We observe that, for a given index $α$ and injection level, the shock modifications are similar for different possible values of $p_{\rm c}$, whereas the particle spectra differ markedly. Of particular interest, low values of $p_{\rm c}$ tend to remove the concavity once thought to be typical of non-linear DSA, and result in steep spectra, as required by recent high-energy observations of Galactic SNRs.

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