[论文解读] Radio Wave Propagation and the Provenance of Fast Radio Bursts

J. M. Cordes, Robert Wharton+3 more|arXiv (Cornell University)|May 19, 2016

Pulsars and Gravitational Waves Research参考文献 7被引用 55

一句话总结

本文通过将快速射电暴（FRBs）与银河系脉冲星进行比较，分析了其色散和散射特性，发现FRBs的色散量（DM）所对应的散射远小于预期。作者得出结论：散射主要由宿主星系引起，而非星际介质（IGM），这意味着若假设IGM主导色散量，则FRB的距离估算值会系统性高估2至3倍。

ABSTRACT

We analyze plasma dispersion and scattering of fast radio bursts (FRBs) to identify the dominant locations of free electrons along their lines of sight and thus constrain the distances of the burst sources themselves. We establish the average $τ$-DM relation for Galactic pulsars and use it as a benchmark for discussing FRB scattering. Though scattering times $τ$ for FRBs are large in the majority of the 17 events we analyze, they are systematically smaller than those of Galactic pulsars that have similar dispersion measures (DMs). The lack of any correlation between $τ$ and DM for FRBs suggests that the intergalactic medium (IGM) cannot account for both $τ$ and DM. We therefore consider mixed models involving the IGM and host galaxies. If the IGM contributes significantly to DM while host galaxies dominate $τ$, the scattering deficit with respect to the mean Galactic trend can be explained with a $τ$-DM relation in the host that matches that for the Milky Way. However, it is possible that hosts dominate both $τ$ and DM, in which case the observed scattering deficits require free electrons in the host to be less turbulent than in the Galaxy, such as if they are in hot rather than warm ionized regions. Our results imply that distances or redshifts of FRB sources can be significantly overestimated if they are based on the assumption that the extragalactic portion of DM is dominated by the IGM.

研究动机与目标

• 确定主导FRB色散和散射的电子含区（银河系、IGM、宿主星系）
• 解决观测到的FRB散射与银河系τ-DM关系之间的矛盾，后者预测在给定DM下应有更高的散射
• 评估散射不足对FRB源距离和红移估算的影响
• 评估IGM或宿主星系是否为主要贡献者，负责FRB的DM和散射

提出的方法

• 利用脉冲星样本建立银河系τ-DM关系，作为散射行为的基准
• 将17个FRB的实测和上限散射时间（τ）与银河系τ-DM趋势进行比较，识别系统性不足
• 使用波动参数F̃量化电子密度湍流，将FRB视线与银河系ISM条件进行比较
• 评估混合模型：IGM贡献DM但宿主星系主导τ，或宿主星系同时主导DM和τ
• 应用来自电浆物理的尺度律，将散射时间τ与电子密度涨落及谱波数分布关联
• 使用文献中关于FRB的DM、τ和银道坐标的数据，计算统计趋势及与银河系基准的偏离

实验结果

研究问题

• RQ1 为何FRBs在色散量（DM）相近时，其脉冲展宽（τ）显著小于银河系脉冲星？
• RQ2 IGM能否同时解释FRBs中观测到的DM和τ？若不能，则需要另一种介质？
• RQ3 若假设IGM主导DM，则散射不足对FRB距离和红移估算有何影响？
• RQ4 宿主星系中的哪些物理条件（如温度、湍流）可解释其τ-DM关系与银河系相比的缺失？
• RQ5 IGM、宿主星系和局部环境的相对贡献如何影响对FRB DM和τ测量的解释？

主要发现

• FRBs作为一个整体，其散射（τ）相对于银河系τ-DM关系存在系统性不足，大多数事件的τ显著小于其DM所预期的值
• 仅靠IGM无法解释该散射不足；相反，宿主星系必须主导散射，且宿主星系中的τ-DM关系与银河系一致
• 若IGM显著贡献DM而宿主星系主导τ，则宿主星系对DM的贡献估计为25%–50%，意味着基于IGM的测距估算会高估2至3倍
• 在另一种模型中，若宿主星系同时主导τ和DM，则宿主星系中的电子密度涨落必须比银河系ISM弱30至60倍，表明其等离子体更热、更少湍流（如T ≳ 10⁶ K）
• FRB010724是唯一在考虑球面波到平面波效应后仍与银河系τ-DM趋势一致的事件，表明其宿主环境可能具有盘状特征
• FRB视线的波动参数F̃通常比银河系小30至60倍，表明散射介质中的湍流被显著抑制