[论文解读] Magnetar emergence in a peculiar gamma-ray burst from a compact star merger

Hui Sun, C. W. Wang+43 more|arXiv (Cornell University)|Jul 11, 2023

Gamma-ray bursts and supernovae被引用 11

一句话总结

这篇论文利用广谱的激发辐射数据分析 GRB 230307A，发现证据表明在激发阶段出现磁星中心引擎并形成窄喷流，支持紧凑恒星合并起源并约束中子星方程状态。

ABSTRACT

The central engine that powers gamma-ray bursts (GRBs), the most powerful explosions in the universe, is still not identified. Besides hyper-accreting black holes, rapidly spinning and highly magnetized neutron stars, known as millisecond magnetars, have been suggested to power both long and short GRBs. The presence of a magnetar engine following compact star mergers is of particular interest as it would provide essential constraints on the poorly understood equation of state for neutron stars. Indirect indications of a magnetar engine in these merger sources have been observed in the form of plateau features present in the X-ray afterglow light curves of some short GRBs. Additionally, some X-ray transients lacking gamma-ray bursts (GRB-less) have been identified as potential magnetar candidates originating from compact star mergers. Nevertheless, smoking gun evidence is still lacking for a magnetar engine in short GRBs, and the associated theoretical challenges have been raised. Here we present a comprehensive analysis of the broad-band prompt emission data of a peculiar, very bright GRB 230307A. Despite its apparently long duration, the prompt emission and host galaxy properties are consistent with a compact star merger origin, as suggested by its association with a kilonova. Intriguingly, an extended X-ray emission component shows up as the $γ$-ray emission dies out, signifying the likely emergence of a magnetar central engine. We also identify an achromatic temporal break in the high-energy band during the prompt emission phase, which was never observed in previous bursts and reveals a narrow jet with half opening angle of approximately $\sim 3.4^\circ (R_{GRB}/10^{15}~{cm})^{-1/2}$, where $R_{GRB}$ is the GRB prompt emission radius.

研究动机与目标

• 推动在紧凑星合并中寻找磁星中心引擎及其对中子星方程状态约束的意义。
• 在广泛能量带内表征激发辐射及早期 X 射线后沿，识别不同的辐射分量。
• 从时间/光谱特征推断中心引擎物理（磁星偶极磁矩自旋减慢）及喷流几何。
• 通过将 GRB 分类为 I 型/II 型以及宿主星系情境来评估其前体性质。

提出的方法

• 使用 GECAM-B/C（10–1000 keV）和 LEIA（0.5–4 keV）数据进行激发辐射的时变光谱分析。
• 识别时间断裂和曲率效应衰减以推断辐射结束与喷流边缘效应。
• 用磁星自旋减速模型拟合 LEIA X 射线光变，以推导偶极磁场与初始自旋周期。
• 从同色破裂推导喷流开口角并计算约束后能量。
• 基于持续时间、最小变异时间尺度、光谱延迟、Amati-type 关系和宿主星系位移等多指标进行 GRB 分类。
• 与 GECAM 和 LEIA 波段的时变联合拟合结果进行交叉验证。

实验结果

研究问题

• RQ1 GRB 230307A 在激发阶段是否出现磁星中心引擎的证据？
• RQ2 喷流特性（开口角、抑郁后能量）是多少，与典型 I 型 GRB 相比如何？
• RQ3 软 X 射线发射是否可由与高能激发辐射不同的磁星自旋减速分量来解释？
• RQ4 爆发特性是否支持紧凑星合并起源并对中子星方程状态进行约束？
• RQ5 GRB 230307A 在 GRB 分类体系（I 型 vs II 型）和宿主星系情境中如何定位？

主要发现

• 后期 LEIA X 射线光变与磁星偶极自旋减速一致，得到磁场约为 ~2.21e16 G，初始自旋周期约为 ~3.49 ms。
• 激发发射表现出无色断裂，指示窄喷流，半开角约 ~3.4 度，抑制校正能量约为 ~5.4e49 erg。
• 爆发的宽带激发数据与宿主星系关联与紧凑星合并起源及 kilonova 徽兆一致。
• 随着伽玛射线发射衰减，出现扩展的软 X 射线发射分量，支持磁星中心引擎而非长寿命吸积盘黑洞引擎。
• GRB 230307A 在持续时间-变异支配的度量和偏移方面符合 I 型 GRB 的特征，为长持续时间的 I 型 GRB 实例之一（与 GRB 211211A 并列）。
• 跨 0.5–6000 keV 的联合光谱分析揭示一个独特的、磁星驱动的分量主导软 X 射线带，光谱特征缓慢演化。