[论文解读] Bright X-ray and Radio Pulses from a Recently Reactivated Magnetar

Aaron B. Pearlman, Walid A. Majid+10 more|arXiv (Cornell University)|May 18, 2020

Pulsars and Gravitational Waves Research参考文献 74被引用 7

一句话总结

本研究报告利用NICER在非耀发状态下探测到最近重新激活的磁星XTE J1810–197发出的明亮、持久的单个X射线脉冲。与以往的突发X射线事件不同，这些脉冲能量较低，具有明显的时序结构，且X射线与无线电波脉冲的幅度或到达时间之间无相关性，表明尽管在8.3和31.9 GHz同时观测，两种波段的辐射机制可能独立。

ABSTRACT

Magnetars are young, rotating neutron stars that possess larger magnetic fields ($B$ $\approx$ $10^{13}$-$10^{15}$ G) and longer rotational periods ($P$ $\approx$ 1-12 s) than ordinary pulsars. In contrast to rotation-powered pulsars, magnetar emission is thought to be fueled by the evolution and decay of their powerful magnetic fields. They display highly variable radio and X-ray emission, but the processes responsible for this behavior remain a mystery. We report the discovery of bright, persistent individual X-ray pulses from XTE J1810-197, a transient radio magnetar, using the Neutron star Interior Composition Explorer (NICER) following its recent radio reactivation. Similar behavior has only been previously observed from a magnetar during short time periods following a giant flare. However, the X-ray pulses presented here were detected outside of a flaring state. They are less energetic and display temporal structure that differs from the impulsive X-ray events previously observed from the magnetar class, such as giant flares and short X-ray bursts. Our high frequency radio observations of the magnetar, carried out simultaneously with the X-ray observations, demonstrate that the relative alignment between the X-ray and radio pulses varies on rotational timescales. No correlation was found between the amplitudes or temporal structure of the X-ray and radio pulses. The magnetar's 8.3 GHz radio pulses displayed frequency structure, which was not observed in the pulses detected simultaneously at 31.9 GHz. Many of the radio pulses were also not detected simultaneously at both frequencies, which indicates that the underlying emission mechanism producing these pulses is not broadband. We find that the radio pulses from XTE J1810-197 share similar characteristics to radio bursts detected from fast radio burst (FRB) sources, some of which are now thought to be produced by active magnetars.

研究动机与目标

• 研究XTE J1810–197在经历十多年的宁静期后重新恢复无线电发射后的X射线与无线电波发射特性。
• 确定新探测到的X射线脉冲是否与巨耀发有关，或代表磁星发射的一种新类型。
• 检查X射线与无线电波脉冲在能量、幅度和时间上的相关性，以约束其辐射机制。
• 表征高频段（8.3和31.9 GHz）的无线电谱和脉冲结构，以评估宽带辐射特性。

提出的方法

• X射线观测使用国际空间站上的中子星内部组成探测器（NICER）于2019年2月6日至26日期间进行，利用同期8.3 GHz定时观测获得的无线电星历表对太阳系质心光子到达时间进行折叠。
• 无线电观测于2019年2月16日和25日在澳大利亚堪培拉的NASA深空网络（DSN）34米天线上，以8.3和31.9 GHz同时进行。
• 通过正弦拟合测量不同能带（1–2、2–3、…、5–10 keV）之间相对相位偏移，分析多个能带中的脉冲轮廓。
• 利用背景扣除后的RMS值计算X射线调制深度，以量化调制深度。
• 从去色散数据生成无线电动态谱和斯托克斯I积分脉冲轮廓，以研究频谱和时序结构。
• 针对MJD 58530.8和MJD 58539.8的8.3 GHz观测，推导出相干时间解，以实现精确的脉冲折叠和相位对齐。

实验结果

研究问题

• RQ1 XTE J1810–197发出的明亮、持久的X射线脉冲是否与巨耀发相关，或是在宁静状态下的一种独立辐射机制？
• RQ2 在相同自转相位下，X射线与无线电波脉冲的时序结构和幅度之间存在何种关系？
• RQ3 8.3和31.9 GHz的无线电波脉冲是否表现出相关发射，表明存在宽带辐射过程？
• RQ4 8.3至31.9 GHz之间的频谱指数与以往测量结果相比如何，其对辐射机制有何启示？
• RQ5 无线电波脉冲中是否存在频率结构，且该结构是否随观测频率变化？

主要发现

• XTE J1810–197的X射线脉冲具有持久性，且在非耀发状态下被探测到，5–10 keV能段的调制深度为0.298 ± 0.009，表明调制强烈。
• 不同能段X射线脉冲轮廓之间的相对相位偏移较小，(4–5)/(5–10) keV能段的相位偏移为–0.025 ± 0.003，表明软X射线波段内存在相干辐射。
• 未发现X射线与无线电波脉冲幅度或时序结构之间存在显著相关性，表明两种波段的辐射机制独立。
• 8.3 GHz无线电波脉冲表现出31.9 GHz未见的频率结构，且多数脉冲未在两个频率上同时被探测到，表明非宽带辐射。
• 8.3至31.9 GHz之间的无线电频谱指数为–0.2 ± 0.2，与平坦谱一致，8.3 GHz处平均通量密度为4.6 ± 0.9 mJy，31.9 GHz处为3.7 ± 0.7 mJy。
• 无线电脉冲特征与快速射电暴（FRBs）相似，尤其体现在其窄带、明亮且具有频谱结构的特性，支持部分FRBs起源于活跃磁星的假设。