High-cadence monitoring of the emission properties of magnetar XTE J1810−197 with the Stockert radio telescope⋆⋆⋆,Astronomy & Astrophysics

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High-cadence monitoring of the emission properties of magnetar XTE J1810−197 with the Stockert radio telescope⋆⋆⋆
Astronomy & Astrophysics ( IF 5.4 ) Pub Date : 2024-06-05 , DOI: 10.1051/0004-6361/202348878
Marlon L. Bause , Wolfgang Herrmann , Laura G. Spitler

Context. Since the detection of a burst resembling a fast radio burst (FRB) from the Galactic magnetar SGR 1935+2154, magnetars have joined the set of favourable candidates for FRB progenitors. However, the emission mechanism of magnetars remains poorly understood.Aims. Observations of magnetars with a high cadence over extended timescales have allowed for their emission properties to be determined, in particular, their temporal variations. In this work, we present the results of the long-term monitoring campaign of the magnetar XTE J1810−197 since its second observed active phase from December 2018 until November 2021, with the Stockert 25 m radio telescope.Methods. We present a single pulse search method, improving on commonly used neural network classifiers thanks to the filtering of radio frequency interference based on its spectral variance and the magnetar’s rotation.Results. With this approach, we were able to lower the signal to noise ratio (S/N) detection threshold from 8 to 5. This allowed us to find over 115 000 spiky single pulses – compared to 56 000 from the neutral network approach. Here, we present the temporal variation of the overall profile and single pulses. Two distinct phases of different single pulse activity can be identified: phase 1 from December 2018 to mid-2019, with a few single pulses per hour, and phase 2 from September 2020 with hundreds of single pulses per hour (with a comparable average flux density). We find that the single pulse properties and folded profile in phase 2 exhibit a change around mid-March 2021. Before this date, the folded profile consists of a single peak and single pulses, with fluences of up to 1000 Jyms and a single-peaked width distribution at around 10 ms. After mid-March 2021, the profile consists of a two peaks and the single pulse population shows a bimodal width distribution with a second peak at 1 ms and fluences of up to 500 Jyms. We also present asymmetries in the phase-resolved single pulse width distributions beginning to appear in 2020, where the pulses arriving earlier in the rotational phase appear wider than those appearing later. This asymmetry persists despite the temporal evolution of the other single pulse and emission properties.Conclusions. We argue that a drift in the emission region in the magnetosphere may explain this observed behaviour. Additionally, we find that the fluence of the detected single pulses depends on the rotational phase and the highest fluence is found in the centre of the peaks in the profile. While the majority of the emission can be linked to the detected single pulses, we cannot exclude another weak mode of emission. In contrast to the pulses from SGR 1935+2154, we have not found any spectral feature or bursts with energies in the order of magnitude of an FRB during our observational campaign. Therefore, the question of whether this magnetar is capable of emitting such highly energetic bursts remains open.

中文翻译：

使用斯托克特射电望远镜对磁星 XTE J1810−197 的发射特性进行高节奏监测⋆⋆⋆

语境。自从从银河磁星 SGR 1935+2154 检测到类似于快速射电暴 (FRB) 的爆发以来，磁星已加入到 FRB 起源的有利候选者行列。然而，人们对磁星的发射机制仍然知之甚少。在延长的时间尺度上对磁星进行高节奏的观测可以确定它们的发射特性，特别是它们的时间变化。在这项工作中，我们介绍了使用斯托克特 25 m 射电望远镜对磁星 XTE J1810−197 自 2018 年 12 月至 2021 年 11 月第二次观测活动阶段以来的长期监测活动的结果。我们提出了一种单脉冲搜索方法，通过基于频谱方差和磁星旋转来过滤射频干扰，改进了常用的神经网络分类器。结果。通过这种方法，我们能够将信噪比 (S/N) 检测阈值从 8 降低到 5。这使我们能够发现超过 115 000 个尖峰单脉冲，而中性网络方法只能找到 56 000 个脉冲。在这里，我们展示了整体轮廓和单个脉冲的时间变化。可以识别不同单脉冲活动的两个不同阶段：阶段 1 从 2018 年 12 月到 2019 年中期，每小时有几个单脉冲，阶段 2 从 2020 年 9 月开始，每小时有数百个单脉冲（具有可比较的平均通量密度））。我们发现，第 2 阶段的单脉冲特性和折叠剖面在 2021 年 3 月中旬左右出现了变化。在此日期之前，折叠剖面由单峰和单脉冲组成，注量高达 1000 Jyms，单峰宽度分布在 10 ms 左右。 2021 年 3 月中旬之后，该曲线由两个峰值组成，单脉冲群显示出双峰宽度分布，第二个峰值位于 1 ms 处，注量高达 500 Jyms。我们还提出了 2020 年开始出现的相位分辨单脉冲宽度分布的不对称性，其中在旋转相位中较早到达的脉冲看起来比较晚出现的脉冲更宽。尽管其他单脉冲和发射特性随着时间的推移，这种不对称性仍然存在。结论。我们认为磁层发射区域的漂移可以解释这种观察到的行为。此外，我们发现检测到的单个脉冲的注量取决于旋转相位，并且最高注量出现在轮廓峰值的中心。虽然大部分发射可以与检测到的单脉冲相关，但我们不能排除另一种弱发射模式。与 SGR 1935+2154 的脉冲相比，我们在观测期间没有发现任何光谱特征或 FRB 数量级的能量爆发。因此，这颗磁星是否能够发射如此高能的爆发的问题仍然悬而未决。