[论文解读] Stringent upper limits on pulsed radio emission during an active bursting phase of the Galactic magnetar SGRJ1935+2154

L. C.-C. Lin, C. F. Zhang+46 more|arXiv (Cornell University)|May 23, 2020

Pulsars and Gravitational Waves Research被引用 9

一句话总结

本研究报告了一项针对银河系磁星SGR J1935+2154在活跃爆发期的八小时FAST射电望远镜观测，期间检测到29次SGR爆发，但未发现任何脉泽射电辐射。未检测到射电辐射表明其辐射通量上限比FRB 200428深八个数量级，暗示FRB与SGR爆发的关联极为罕见，且FRB可能具有高度束射性、谱宽极窄，或需要极端物理条件才能产生相干辐射。

ABSTRACT

Fast radio bursts (FRBs) are mysterious millisecond-duration radio transients observed from extragalactic distances. Magnetars have been long speculated as the possible engine to power repeating bursts from FRB sources, but no convincing evidence has been collected so far. Recently, a Galactic magnetar dubbed Soft Gamma-ray Repeater (SGR) J1935+2154 entered an active phase by emitting intense soft gamma-ray bursts. One fast radio burst with two peaks (hereafter FRB 200428) and a luminosity comparable to those of extragalactic FRBs was detected from the source in association with a soft $\gamma$-ray / hard X-ray flare. Here we report an eight-hour targeted observational campaign with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in four observing sessions, assisted by multi-wavelength (optical and hard X-rays) observations. During the third session, twenty-nine SGR bursts were detected by the {\it Fermi} Gamma-ray Burst Monitor (GBM). Throughout the observing period, especially during the scrutinized epochs when SGR bursts reached the Earth, no single dispersed pulsed emission was detected by FAST. This places a fluence upper limit eight orders of magnitude deeper than the fluence of FRB 200428. Our results suggest that FRB -- SGR burst associations are rather rare. FRBs may be highly relativistic and geometrically beamed or may carry narrow spectra with the characteristic frequencies of the most bursts far outside the FAST band. More intriguingly, it is also possible that the physical condition to achieve coherent radiation in SGR bursts is contrived. Only under extreme conditions could an FRB be made in association with an SGR burst.

研究动机与目标

• 检验磁星SGR爆发在活跃期是否能产生快速射电暴（FRB）的假设。
• 探究FRB 200428型辐射是否与银河系磁星SGR J1935+2154的SGR爆发相关。
• 通过在强烈爆发活动期间搜索弥散化、脉冲式射电辐射，约束SGR爆发的射电辐射特性。
• 确定FRB产生是否为SGR爆发的普遍结果，还是需要罕见且极端的物理条件。

提出的方法

• 利用五百米口径球面射电望远镜（FAST）在SGR J1935+2154的活跃期开展为期八小时的定向射电观测活动。
• 分四次观测时段进行FAST观测，与*Fermi*伽马射暴监视器（GBM）及光学/硬X射线观测的多波段数据同步。
• 在SGR爆发活动频繁的时段监测弥散化、脉冲式射电辐射，特别是在第三次观测时段（检测到29次SGR爆发）重点分析。
• 应用高灵敏度搜索算法，检测具有亚毫秒 timescale 的相干、弥散射电脉冲，特征与FRB一致。
• 通过将检测阈值与FRB 200428的通量进行比较，计算脉冲射电辐射的通量上限。
• 利用未检测到信号的结果，推导出SGR爆发能量在射电波段辐射比例的严格上限。

实验结果

研究问题

• RQ1 在SGR J1935+2154活跃期，SGR爆发是否伴随可探测的脉冲射电辐射？
• RQ2 SGR爆发的射电通量与唯一已知与磁星相关的FRB 200428相比如何？
• RQ3 非检测结果对FRB与磁星活动关联的模型有何影响？
• RQ4 FRB辐射是否可能高度束射、谱宽极窄，或需要极端物理条件才能产生？
• RQ5 FRB与SGR爆发的关联是普遍现象，还是罕见的特殊事件？

主要发现

• 尽管*Fermi* GBM检测到29次SGR爆发，FAST在八小时观测期间未检测到任何脉冲射电辐射。
• 脉冲射电辐射的通量上限比FRB 200428的通量深八个数量级，表明射电辐射效率极低。
• 未检测到信号意味着FRB–SGR爆发关联可能极为罕见，挑战了所有SGR爆发均能产生FRB的观点。
• 结果表明FRB可能具有高度相对论性和几何束射特性，或具有超出FAST观测波段的窄谱。
• SGR爆发中产生相干射电辐射的物理条件可能极为特殊，需极端且罕见的构型。
• 仅在极端条件下，FRB才可能与SGR爆发相关联，表明此类事件并非磁星活动的普遍结果。