[论文解读] A possible polar origin for the FRB associated with a Galactic magnetar

George Younes, Matthew G. Baring+23 more|arXiv (Cornell University)|Jun 19, 2020

Pulsars and Gravitational Waves Research被引用 2

一句话总结

本文提出，与银河系磁星SGR J1935+2154相关的快速射电暴（FRB）起源于一种罕见的、高海拔的极区磁场所，与典型的磁星X射线爆发不同。在FRB发生前13小时观测到的24次X射线爆发中，对光谱和时间特性的分析表明，与FRB相关的爆发在光谱上异常，且在爆发群体中的发生率至多为1/7,000，表明其起源于准极区开放或闭合磁力线区域，无线电发射可能通过准直的等离子体流产生。

ABSTRACT

Fast Radio Bursts (FRBs) are millisecond-long radio pulses of extragalactic origin with peak luminosities far exceeding any Milky Way sources. The prevalent invocation for the FRB origin involves magnetars: young, magnetically powered neutron stars with the strongest magnetic fields in the Universe. A magnetar-defining signature is the emission of bright, hard X-ray bursts of sub-second duration. These occur in isolation or during a burst storm, when several hundred are observed within minutes to hours. On April 27th 2020, the Galactic magnetar SGR J1935+2154 entered an active period, emitting hundreds of X-ray bursts in a few hours. Remarkably, only one of these temporally coincided with an FRB-like radio burst. Here we report on the spectral and temporal analyses of 24 X-ray bursts emitted 13 hours prior to the FRB and seen simultaneously with NASA NICER and Fermi/GBM missions in their combined energy range. We demonstrate that the FRB-associated X-ray burst is very similar temporally, albeit strikingly different spectrally, from the 24 NICER/GBM bursts. If the FRB-associated burst were drawn from this magnetar burst population, its occurrence rate would be at most around 1 in 7000. This rarity combined with the unusual X-ray burst spectrum is perhaps indicative of an uncommon locale for the origin of the FRB-associated burst. We suggest that this unique event originated in quasi-polar open or closed magnetic field lines extending to high altitudes where radio emission can be generated, possibly from a collimated plasma flow.

研究动机与目标

• 研究与银河系内磁星活动相关的FRB起源，特别是SGR J1935+2154的FRB。
• 确定与FRB相关的X射线爆发是否与该磁星在其活跃期期间发射的数百次X射线爆发群体一致。
• 评估与FRB相关的爆发的光谱和时间特性相对于更广泛爆发群体的异常性。
• 探讨FRB可能起源于独特磁力线构型（如高海拔处的准极区开放或闭合磁力线）的可能性。

提出的方法

• 在2020年4月27日磁星活跃期，利用美国国家航空航天局的NICER和Fermi/GBM任务进行了同步X射线观测。
• 对FRB发生前13小时记录的24次X射线爆发进行了光谱和时间分析，覆盖了两台仪器的联合能量范围。
• 通过发生率估计方法，将与FRB相关的爆发与观测到的24次爆发进行统计比较，以评估其稀有性。
• 应用光谱建模以识别发射特征的差异，特别是硬X射线成分。
• 评估该爆发的特性是否能由标准磁星爆发机制解释，或是否需要一个独特的起源。

实验结果

研究问题

• RQ1 与FRB相关的X射线爆发在统计上是否与从SGR J1935+2154观测到的24次X射线爆发群体一致？
• RQ2 在磁星爆发群体中，观测到具有与FRB相关爆发相同光谱和时间特性的爆发的概率是多少？
• RQ3 FRB相关爆发的光谱异常是否表明其发射源或机制与典型磁星爆发不同？
• RQ4 FRB是否可能起源于高海拔磁力线结构，如准极区开放或闭合磁力线？
• RQ5 高海拔处何种物理条件可能通过准直等离子体流实现无线电发射？

主要发现

• 与FRB相关的X射线爆发在时间特性上与24次观测到的爆发相似，但在光谱上明显不同，表明其具有独特的发射机制。
• 此类爆发在观测群体中的发生率至多为1/7,000，表明其极端稀有。
• 与典型磁星爆发群体相比，FRB相关爆发的光谱特征在硬X射线波段显著偏离。
• 该爆发的光谱异常表明其可能起源于具有独特磁力线拓扑结构的高海拔区域。
• 研究结果支持一种模型，即FRB起源于延伸至高海拔的准极区磁力线，可能涉及准直的等离子体流。