[论文解读] Relaxation of Blazar Induced Pair Beams in Cosmic Voids: Measurement of Magnetic Field in Voids and Thermal History of the IGM

Francesco Miniati, A. Elyiv|arXiv (Cornell University)|Aug 8, 2012

Astrophysics and Cosmic Phenomena被引用 3

一句话总结

本研究探讨了由星系系中类星体产生的TeV伽马射线在宇宙空洞中生成的超相对论性电子-正电子束的稳定性。通过蒙特卡洛模拟与 kinetic 理论，发现非线性朗道阻尼和密度非均匀性会抑制集体振荡的增长，导致束流弛豫 timescale 远长于逆康普顿损耗 timescale，表明该机制对星际介质热历史或磁场测量的影响可忽略不计。

ABSTRACT

The stability properties of a low density ultra relativistic pair beam produced in the intergalactic medium by multi-TeV gamma-ray photons from blazars are analyzed. The problem is relevant for probes of magnetic field in cosmic voids through gamma-ray observations. In addition, dissipation of such beams could affect considerably the thermal history of the intergalactic medium and structure formation. We use a Monte Carlo method to quantify the properties of the blazar induced electromagnetic shower, in particular the bulk Lorentz factor and the angular spread of the pair beam generated by the shower, as a function of distance from the blazar itself. We then use linear and nonlinear kinetic theory to study the stability of the pair beam against the growth of electrostatic plasma waves, employing the Monte Carlo results for our quantitative estimates. We find that the fastest growing mode, like any perturbation mode with even a very modest component perpendicular to the beam direction, cannot be described in the reactive regime. Due to the effect of non-linear Landau damping, which suppresses the growth of plasma oscillations, the beam relaxation timescale is found significantly longer than the inverse Compton loss time. Finally, density inhomogeneities associated with cosmic structure induce loss of resonance between the beam particles and plasma oscillations, strongly inhibiting their growth. We conclude that relativistic pair beams produced by blazars in the intergalactic medium are stable on timescales long compared to the electromagnetic cascade's. There appears to be little or no effect of pair-beams on the intergalactic medium.

研究动机与目标

• 评估由类星体发出的多TeV伽马射线在星际介质（IGM）中产生的相对论性对束的稳定性。
• 评估此类束流通过伽马射线观测探测宇宙空洞中磁场的潜力。
• 研究束流耗散是否可能显著改变IGM的热历史或影响结构形成。
• 量化非线性朗道阻尼和密度非均匀性在抑制集体振荡增长中的作用。
• 确定对束流的弛豫 timescale 相对于电磁级联 timescale 的长短。

提出的方法

• 采用蒙特卡洛方法模拟TeV伽马射线产生的电磁级联过程，追踪所生成对束的平均洛伦兹因子和角展宽。
• 利用线性和非线性 kinetic 理论，模拟在相对论性对束存在下静电集体振荡的增长。
• 将蒙特卡洛模拟结果作为输入，对束流稳定性与波增长速率进行定量估算。
• 分析非线性朗道阻尼对束流参考系中集体振荡抑制的影响。
• 评估由宇宙结构引起的密度非均匀性如何破坏束流粒子与集体振荡之间的共振。
• 将束流弛豫 timescale 与逆康普顿损耗 timescale 进行比较，以评估束流的持久性及其潜在影响。

实验结果

研究问题

• RQ1 由类星体伽马射线在宇宙空洞中产生的相对论性对束是否能在与IGM热演化相关的 timescale 内保持稳定？
• RQ2 非线性朗道阻尼在束流参考系中对静电集体振荡增长的抑制程度如何？
• RQ3 星际介质中的密度非均匀性如何影响束流粒子与集体振荡之间的共振？
• RQ4 束流弛豫 timescale 是否长于逆康普顿损耗 timescale，表明能量耗散极小？
• RQ5 此类对束是否可通过伽马射线观测有效探测宇宙空洞中的磁场？

主要发现

• 由于存在显著的横向分量，最快增长的静电集体振荡模式无法用反应区理论描述，导致强烈的非线性效应。
• 非线性朗道阻尼强烈抑制集体振荡的增长，导致束流弛豫 timescale 显著长于逆康普顿损耗 timescale。
• 与宇宙结构相关的密度非均匀性会破坏束流粒子与集体振荡之间的共振，进一步抑制波的增长。
• 束流在远长于电磁级联 timescale 的 timescale 内保持稳定，表明能量耗散极小。
• 因此，这些对束对星际介质热历史几乎或完全没有可探测的影响。
• 该机制无法为通过束流驱动过程探测宇宙空洞中的磁场提供可行途径。