[论文解读] Radio properties of the optically identified supernova remnant G107.0+9.0

W. Reich, X. Gao+1 more|arXiv (Cornell University)|Aug 19, 2021

Astrophysics and Cosmic Phenomena参考文献 38被引用 2

一句话总结

本研究通过22 MHz至4.8 GHz的多频射电观测，确认G107.0+9.0为一个高度演化、直径较大（75–100 pc）、表面亮度较低的超新星遗迹（SNR）。观测检测到具有谱指数α = −0.95 ± 0.04的陡谱非热射电辐射，并通过去偏振分析在西部边缘区域识别出约15 µG的压缩磁场，表明尽管存在混淆，仍可从大尺度银河系射电发射中提取出微弱且延伸的SNR。

ABSTRACT

The vast majority of Galactic supernova remnants (SNRs) were detected by their synchrotron radio emission. Recently, the evolved SNR G107.0+9.0 with a diameter of about 3~deg or 75~pc up to 100~pc in size was optically detected with an indication of faint associated radio emission. This SNR requires a detailed radio study. We aim to search for radio emission from SNR G107.0+9.0 by analysing new data from the Effelsberg 100-m and the Urumqi 25-m radio telescopes in addition to available radio surveys. Radio SNRs outside of the Galactic plane, where confusion is rare, must be very faint if they have not been identified so far. Guided by the H$\alpha$ emission of G107.0+9.0, we separated its radio emission from the Galactic large-scale emission. Radio emission from SNR G107.0+9.0 is detected between 22~MHz and 4.8~GHz with a steep non-thermal spectrum, which confirms G107.0+9.0 as an SNR. Its surface brightness is among the lowest known for Galactic SNRs. Polarised emission is clearly detected at 1.4~GHz but is fainter at 4.8~GHz. We interpret the polarised emission as being caused by a Faraday screen associated with G107.0+9.0 and its surroundings. Its ordered magnetic field along the line of sight is below 1~$\mu$G. At 4.8~GHz, we identified a depolarised filament along the western periphery of G107.0+9.0 with a magnetic field strength along the line of sight $B{_{||}} \sim 15~\mu$G, which requires magnetic field compression. G107.0+9.0 adds to the currently small number of known, evolved, large-diameter, low-surface-brightness Galactic SNRs. We have shown that such objects can be successfully extracted from radio-continuum surveys despite the dominating large-scale diffuse Galactic emission.

研究动机与目标

• 检测并表征光学已识别、大直径SNR G107.0+9.0的微弱射电辐射，该天体此前未在射电巡天中被识别。
• 确定G107.0+9.0的射电谱特性，并通过非热辐射特征确认其作为超新星遗迹的性质。
• 分析偏振射电辐射，推断遗迹及其周围区域的磁场结构，包括法拉第屏效应和磁场压缩。
• 证明尽管存在主导的弥散发射，仍可成功从大尺度银河系射电巡天中提取出低表面亮度、高度演化的SNR。

提出的方法

• 利用Finkbeiner (2003)的Hα发射图确定感兴趣区域，并指导射电辐射的提取。
• 应用非锐化掩模和背景滤波技术，从22 MHz和408 MHz的弥散银河背景中分离出G107.0+9.0的射电辐射。
• 结合埃菲尔斯堡100米和乌鲁木齐25米望远镜在1.4 GHz和4.8 GHz的观测数据，包含线性偏振信息。
• 对SNR区域进行流量积分，推导出总射电谱和谱指数。
• 应用法拉第屏（FS）模型解释偏振辐射，估算旋转量（RM）、磁场强度（B||）和填充因子。
• 使用包含参数（PAon − PAoff, PIon/PIoff, RM）的法拉第屏模型，推断视线方向的本征磁场强度和几何结构。

实验结果

研究问题

• RQ1 尽管存在主导的弥散银河系发射，像G107.0+9.0这样的微弱、大直径、高度演化的SNR是否能在射电巡天中被探测到？
• RQ2 G107.0+9.0的谱指数是多少？是否确认其具有与SNR一致的非热、同步辐射起源？
• RQ3 视线方向的磁场结构如何？偏振辐射是否表明磁场压缩或存在法拉第屏？
• RQ4 在4.8 GHz处的去偏振现象如何反映G107.0+9.0西部边缘区域的物理条件？

主要发现

• G107.0+9.0表现出陡峭的非热射电谱，总谱指数为α = −0.95 ± 0.04，确认其为超新星遗迹的性质。
• G107.0+9.0的表面亮度在1 GHz时约为5.4 × 10−23 W m−2 Hz−1 sr−1，为银河系SNR中已知最低之一。
• 在1.4 GHz处清晰检测到偏振辐射，4.8 GHz处较弱，解释为来自视线方向有序磁场（B|| < 1 µG）的法拉第屏。
• 在4.8 GHz处沿G107.0+9.0西部边缘检测到去偏振丝状结构，表明存在约15 µG的压缩有序磁场，需磁场压缩机制。
• 法拉第屏模型得出去偏振丝状结构的旋转量（RM）约为−300 rad m−2，与共同距离和磁场结构一致。
• 同步辐射发射率估算表明银河系局部发射率增强，距离1.5 kpc时为0.94 K Tb/kpc，距离2 kpc时为0.70 K Tb/kpc，与本地星际介质中已知增强一致。