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arXiv 1306.6454astro-ph.HE
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超新星残余激波中的宽巴耳末线发射和cosmic ray加速效率

Broad Balmer line emission and cosmic ray acceleration efficiency in supernova remnant shocks

G. Morlino, P. Blasi, R. Bandiera, E. Amato

摘要:

巴耳末辐射可能是一个强有力的诊断工具,以测试宇宙线(CR)加速的范式在年轻超新星遗迹(SNR)冲击。宽巴耳末线的宽度是下游等离子体温度的直接指标。在有效粒子加速的情况下,等离子体的总动能的相当大的一部分被引导到CR中,因此下游温度降低,宽巴耳末线宽度也降低。该宽度还取决于下游离子和中性氢原子之间的热平衡水平。由于一般在年轻的SNR冲击只有少数电荷交换(CE)反应发生电离前,离子和中性之间的平衡没有达到,和中性的动力学描述是必需的,以正确计算巴耳末发射。我们提供了一种方法,用于计算巴耳末发射使用的冲击结构的自洽描述中存在的中性和CR。我们使用一个最近开发的半解析方法,中性粒子,电离等离子体,加速粒子和磁场都耦合在一起,通过质量,动量和能量通量守恒方程。中性粒子的分布是从速度空间中的完整玻尔兹曼方程得到的,通过电离和CE过程耦合到麦克斯韦离子。由于原子相互作用速率的更好近似,相对于以前的工作,计算也得到了改进。我们发现,当激波速度大于2500 km/s时,宽中性粒子的分布永远不会接近麦克斯韦分布,其矩也不同于电离分量的矩。这些差异反映到一个较小的FWHM比在以前的计算中预测,热化假设。这里提出的方法提供了一个现实的估计粒子加速效率在Balmer主导的冲击。

英文摘要:

Balmer emission may be a powerful diagnostic tool to test the paradigm of cosmic ray (CR) acceleration in young supernova remnant (SNR) shocks. The width of the broad Balmer line is a direct indicator of the downstream plasma temperature. In case of efficient particle acceleration an appreciable fraction of the total kinetic energy of the plasma is channeled into CRs, therefore the downstream temperature decreases and so does the broad Balmer line width. This width also depends on the level of thermal equilibration between ions and neutral hydrogen atoms in the downstream. Since in general in young SNR shocks only a few charge exchange (CE) reactions occur before ionization, equilibration between ions and neutrals is not reached, and a kinetic description of the neutrals is required in order to properly compute Balmer emission. We provide a method for the calculation of Balmer emission using a self-consistent description of the shock structure in the presence of neutrals and CRs. We use a recently developed semi-analytical approach, where neutral particles, ionized plasma, accelerated particles and magnetic fields are all coupled together through the mass, momentum and energy flux conservation equations. The distribution of neutrals is obtained from the full Boltzmann equation in velocity space, coupled to Maxwellian ions through ionization and CE processes. The computation is also improved with respect to previous work thanks to a better approximation for the atomic interaction rates. We find that for shock speeds >2500km/s the distribution of broad neutrals never approaches a Maxwellian and its moments differ from those of the ionized component. These differences reflect into a smaller FWHM than predicted in previous calculations, where thermalization was assumed. The method presented here provides a realistic estimate of particle acceleration efficiency in Balmer dominated shocks.

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