FRB 20201124A 中的活动转变：方法论的严谨性、频率依赖停止的探测以及几何磁星模型
An activity transition in FRB 20201124A: methodological rigor, detection of frequency-dependent cessation, and a geometric magnetar model
AI总结：
本研究通过Apertif/WSRT和GMRT观测重复暴FRB 20201124A，发现其活动存在频率依赖停止，提出超长周期磁星几何模型，并改进DM和流量分析方法。
摘要：
我们报告了使用 Apertif/WSRT 和 GMRT 对重复暴源 FRB 20201124A 的快速射电暴（FRB）探测，以及对基本暴性质（尤其是色散量（DM）和流量）的测量。基于将这些性质与先前发表的更大样本进行比较，我们认为，对于综合信噪比 $\lesssim 1000$ 的脉冲，先前报道的过量 DM 是由于未完全考虑所谓的“悲伤长号效应”，即使使用了结构最大化 DM 算法也是如此。接下来，我们对流量分布的调查促使我们建议不要进行正式幂律拟合，尤其是不鼓励使用最小二乘法，并展示了其中涉及的大偏差。最大似然估计器（MLE）能提供更准确的幂律估计，我们提供了可直接用于未来研究的易用代码。我们的 GMRT 观测幸运地安排在 FAST 记录的活动周期结束前后。我们在 400/600 MHz 频率下探测到了几个暴（其中一个非常强），而在此几小时前，灵敏的 FAST 非探测已经表明 1.3 GHz 的 FRB 发射已经停止。继 FRB 20180916B 之后，这是在重复暴源中识别出的第二个频率依赖活动窗口的例子。由于众多努力迄今未能确定 FRB 20201124A 的自旋周期，我们推测它是一颗超长周期磁星，周期在月量级，且具有非常宽、极不规则的占空比。假设发射来自闭合场线，我们利用半径-频率映射和其他研究中的偏振信息来约束磁层几何和发射区域的位置。我们的初步发现与 FRB 与地壳运动事件之间可能存在联系相一致。
英文摘要：
We report detections of fast radio bursts (FRBs) from the repeating source FRB 20201124A with Apertif/WSRT and GMRT, and measurements of basic burst properties, especially the dispersion measure (DM) and fluence. Based on comparisons of these properties with previously published larger samples, we argue that the excess DM reported earlier for pulses with integrated signal to noise ratio $\lesssim 1000$ is due to incompletely accounting for the so-called sad trombone effect, even when using structure-maximizing DM algorithms. Our investigations of fluence distributions next lead us to advise against formal power-law fitting, especially dissuading the use of the least-square method, and we demonstrate the large biases involved. A maximum likelihood estimator (MLE) provides a much more accurate estimate of the power law and we provide accessible code for direct inclusion in future research. Our GMRT observations were fortuitously scheduled around the end of the activity cycle as recorded by FAST. We detected several bursts (one of them very strong) at 400/600 MHz, a few hours after sensitive FAST non-detections already showed the 1.3 GHz FRB emission to have ceased. After FRB 20180916B, this is a second example of a frequency-dependent activity window identified in a repeating FRB source. Since numerous efforts have so-far failed to determine a spin period for FRB 20201124A, we conjecture it to be an ultra-long period magnetar, with a period on the scale of months, and with a very wide, highly irregular duty cycle. Assuming the emission comes from closed field lines, we use radius-to-frequency mapping and polarization information from other studies to constrain the magnetospheric geometry and location of the emission region. Our initial findings are consistent with a possible connection between FRBs and crustal motion events.
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