[论文解读] Positrons from pulsar winds

Pasquale Blasi, Elena Amato|arXiv (Cornell University)|Jul 27, 2010

Astrophysics and Cosmic Phenomena参考文献 4被引用 12

一句话总结

本文提出，脉冲星风星云，尤其是形成弓形激波的高速脉冲星周围的星云，是银河系正电子的主要来源。研究表明，这些脉冲星在逃离超新星遗迹后，可通过终止激波特别高效地加速电子-正电子对，当磁矩制动指数为2.5时，效率可达30–50%，无需引入暗物质或奇异源即可自然解释PAMELA探测到的正电子过剩。

ABSTRACT

Pulsars, or more generally rotation powered neutron stars, are excellent factories of antimatter in the Galaxy, in the form of pairs of electrons and positrons. Electrons are initially extracted from the surface of the star by the intense rotation induced electric fields and later transformed into electron-positron pairs through electromagnetic cascading. Observations of Pulsar Wind Nebulae (PWNe) show that cascades in the pulsar magnetosphere must ensure pair multiplicities of order $10^{4}-10^{5}$. These pairs finally end up as part of the relativistic magnetized wind emanating from the pulsar. The wind is slowed down, from its highly relativistic bulk motion, at a termination shock, which represents the reverse shock due to its interaction with the surrounding ejecta of the progenitor supernova. At the (relativistic) termination shock, acceleration of the pairs occurs, as part of the dissipation process, so that the cold wind is transformed into a plasma of relativistic non-thermal particles, plus a potential thermal component, which however has never been observed. As long as the pulsar wind is embedded in the supernova remnant these pairs are forced to escavate a bubble and lose energy adiabatically (because of the expansion) and radiatively (because of magnetic and radiation fields). We discuss here the observational constraints on the energy and number content of such pairs and discuss the scenarios that may allow for the pairs to escape in the interstellar medium and possibly contribute to the positron excess that has recently been detected by the PAMELA satellite. Special attention is dedicated to the case of Pulsar Bow Shock Nebulae. The pairs produced in these objects may be effectively carried out of the Supernova Remnant and released in the Interstellar Medium. As a result, Bow Shock Pulsar Wind Nebulae might be the main contributors to the positron excess in the Galaxy.

研究动机与目标

• 研究脉冲星，特别是高速脉冲星，是否能成为宇宙射线正电子的重要来源。
• 评估脉冲星风星云作为PAMELA探测到的正电子过剩观测结果之来源的可行性。
• 确定脉冲星脱离超新星遗迹后，正电子产生与逃逸的效率。
• 用单一的天体物理源模型协调观测到的电子与正电子能谱。
• 解释为何未观测到反质子过剩，而若正电子源为高能粒子加速器或暗物质衰变则应预期存在反质子过剩。

提出的方法

• 通过电磁级联过程，模拟脉冲星磁层中产生的电子-正电子对的能量与粒子含量。
• 模拟脉冲星相对论性风与超新星喷出物相互作用的动力学，形成加速对粒子的终止激波。
• 分析对粒子在脉冲星风星云扩张空腔中的演化，考虑绝热与辐射能量损失。
• 评估对粒子因弓形激波星云开放、无约束的几何结构，在脉冲星离开遗迹后逃逸至星际介质的效率。
• 利用时间与空间平均的超新星爆发率，计算注入星际介质的电子与正电子通量。
• 结合超新星前向激波与脉冲星风贡献，采用平坦的注入谱，拟合Fermi与PAMELA观测到的电子与正电子能谱。

实验结果

研究问题

• RQ1 脉冲星，特别是高速脉冲星，是否能产生足够多的正电子以解释PAMELA探测到的正电子过剩？
• RQ2 脉冲星脱离其超新星遗迹后，脉冲星风星云中正电子产生与逃逸的效率如何？
• RQ3 脉冲星产生的电子与正电子能谱形状与Fermi和PAMELA的观测结果相比如何？
• RQ4 尽管存在高能正电子源的可能，为何未观测到反质子过剩？
• RQ5 脉冲星风模型是否能同时与电子和正电子能谱一致？

主要发现

• 对于制动指数n=2.5的脉冲星，仅需30–50%的效率即可同时解释Fermi电子谱与PAMELA正电子比。
• 对于偶极型自旋下降（n=3）的情况，所需效率仅约1%，这过于微小，暗示需要更高效率的机制。
• 弓形激波星云的开放几何结构使相对论性对粒子能高效逃逸至星际介质，从而产生可观测的宇宙射线贡献。
• 该模型自然解释了PWNe中观测到的平坦注入谱（E⁻¹至E⁻¹.⁸），并无需引入暗物质即可匹配PAMELA正电子分数。
• 脉冲星风星云与超新星前向激波的联合贡献，能拟合宽能量范围内观测到的电子谱。
• 反质子过剩的缺失可自然解释，因为相同的加速机制不会产生显著的反质子通量。