[论文解读] Constraining the long-lived supramassive neutron stars by magnetar boosted kilonovae

Hao Wang, Paz Beniamini+1 more|arXiv (Cornell University)|Aug 17, 2023

Gamma-ray bursts and supernovaePhysics and Astronomy参考文献 89被引用 3

一句话总结

本文建模了磁星助推型千新星——这类极为明亮的瞬变源由双中子星并合后形成的长寿命超质量中子星（SMNS）的能量注入所驱动。通过详细模拟磁星自转减速、喷流演化及X射线光致电离过程，作者表明，此类事件未被观测到的事实意味着SMNS的能量注入远低于理论极限，要求其在数秒至数分钟内坍缩，从而限制了SMNS的寿命极短，且在宇宙中极为罕见。

ABSTRACT

Kilonovae are optical transients following the merger of neutron star binaries, which are powered by the r-process heating of merger ejecta. However, if a merger remnant is a long-lived supramassive neutron star supported by its uniform rotation, it will inject energy into the ejecta through spindown power. The energy injection can boost the peak luminosity of a kilonova by many orders of magnitudes, thus significantly increasing the detectable volume. Therefore, even if such events are only a small fraction of the kilonovae population, they could dominate the detection rates. However, after many years of optical sky surveys, no such event has been confirmed. In this work, we build a boosted kilonova model with rich physical details, including the description of the evolution and stability of a proto neutron star, and the energy absorption through X-ray photoionization. We simulate the observation prospects and find the only way to match the absence of detection is to limit the energy injection by the newly born magnetar to only a small fraction of the neutron star rotational energy, thus they should collapse soon after the merger. Our result indicates that most supramassive neutron stars resulting from binary neutron star mergers are short lived and they are likely to be rare in the Universe.

研究动机与目标

• 理解磁星助推型千新星的可探测性，这类事件因长寿命超质量中子星（SMNS）的能量注入，其亮度可能比标准千新星高出数个数量级。
• 解决此类事件潜在可观测体积极大与多年光学巡天中未被探测到之间的明显矛盾。
• 利用未观测到此类事件作为关键观测限制，约束新生磁星向喷流注入能量的效率。
• 确定对双中子星并合残余物中物态方程和中心引擎物理的影响。

提出的方法

• 开发了一套详细的助推型千新星模型，整合了原初磁星的自转减速演化，包括通过磁偶极辐射造成的旋转能量损失。
• 追踪了通过X射线光致电离向喷流注入能量的过程，考虑了辐射转移和电离态演化。
• 模拟了光学与近红外波段的光曲线和光谱能量分布，包括喷流膨胀与冷却的影响。
• 通过将模型预测与深光学巡天中未探测到此类事件的结果对比，约束了能量注入比例。
• 对探测率进行了模型依赖与模型无关的分析，以评估未探测到事件的统计显著性。
• 纳入了如磁星喷流泡中瑞利-泰勒不稳定性增长等不稳定性机制，以评估激波形成与余晖辐射。

Figure 1: Illustration of the structure of the post-merger system. The system is composed by an inner pulsar wind nebula (PWN) inflated by a magnetar, and an outer ejecta shell. The PWN is composed by X-ray photons and electron-positron pairs. The X-ray radiation ionize and heat the ejecta, leading

实验结果

研究问题

• RQ1 新生磁星的多少比例旋转能量必须注入喷流，才能产生可探测的磁星助推型千新星？
• RQ2 尽管其潜在可观测体积巨大，为何多年来未探测到任何磁星助推型千新星？
• RQ3 超质量中子星的稳定性和演化如何影响助推型千新星的亮度与光曲线？
• RQ4 未探测到此类事件对长寿命超质量中子星的寿命与形成率有何影响？
• RQ5 磁星喷流从喷流泡中突破进入喷流是否能解释孤儿余晖的缺失？

主要发现

• 未探测到磁星助推型千新星表明，只有极小比例——低于1%——的磁星旋转能量能被注入喷流，排除了高效能量传递的可能性。
• 双中子星并合形成的超质量中子星必须在形成后数秒至数分钟内坍缩，表明其寿命极短，无法长期稳定存在。
• 模型预测表明，助推型千新星的峰值亮度对SMNS存活 timescale 极为敏感，存活时间越长，瞬变体越明亮且越易探测。
• 未探测到激波突破引发的孤儿余晖，表明此类事件极为罕见，进一步支持SMNS快速坍缩的结论。
• 长寿命且快速旋转的SMNS的引力波辐射不太可能被探测到，因为其所需的椭球度与时间尺度与观测到的未探测结果不一致。
• 结果表明，SMNS在宇宙中形成极为罕见，因为只有极少数并合事件能产生这种短寿命残余物。

Figure 2: The Monte Carlo simulation result of absorption, reflection, and transmission rate of a slab caused by both absorption and scattering processes. The red lines show the contour plot of $A^{\rm tra}$ as a function of optical depth of two processes, and the blue lines are the $A^{\rm ref}$ .