[论文解读] Low Frequency Absorption in Cassiopeia A

Maria Arias, Jacco Vink+4 more|arXiv (Cornell University)|Jan 15, 2018

Astrophysics and Cosmic Phenomena被引用 3

一句话总结

利用LOFAR和VLA在30–77 MHz及L波段的射电观测，该研究通过建模英仙座A中的低频自由-自由吸收，以约束未激波喷出物的质量、温度和团块化程度。研究发现，在100 K时质量为$2.95 \pm 0.48\,M_\odot$，较低的温度或更高的团块化程度会降低推断出的质量，且表明长期衰减无法仅通过质量损失来解释，必须存在高团块化或低温气体（$\sim10$ K）才能实现高吸收。

ABSTRACT

Cassiopeia A is one of the best studied supernova remnants. Its shocked ejecta emits brightly in radio and X-rays. Its unshocked ejecta can be studied through infrared emission, the radio-active decay of $^{44}$Ti, and low frequency free-free absorption due to cold gas internal to the shell. Free-free absorption is affected by the mass, geometry, temperature, and ionisation conditions in the absorbing gas. Observations at the lowest radio frequencies constrain a combination of these properties. We use LOFAR LBA observations at 30-77 MHz and L-band VLA observations to compare $u-v$-matched images with a common resolution of 17. We simultaneously fit, per pixel, for the emission measure and the ratio of the emission from the unabsorbed front of the shell versus the absorbed back of the shell. We explore the effects that low temperatures and a high degree of clumping can have on the derived physical properties, such as mass and density. We also compile published radio flux measurements, fit for the absorption processes that occur in the radio band, and consider how they affect the secular decline of the source. We find a mass in the unshocked ejecta of $M = 2.95 \pm {0.48} \,M_{\odot}$ for an assumed gas temperature of $T=100$ K. This estimate is reduced for colder gas temperatures and if the ejecta are clumped. We measure the reverse shock to have a radius of $114$ $\pm $6. We also find that a decrease in the amount of mass in the unshocked ejecta (as more and more material meets the reverse shock and heats up) cannot account for the observed low frequency behaviour of the secular decline rate. To reconcile our low frequency absorption measurements with models that predict little mass in the unshocked ejecta we need the ejecta to be very clumped, or the temperature in the cold gas to be low ($\sim10$ K). Both conditions can jointly contribute to the high absorption.

研究动机与目标

• 利用低频射电吸收测量，确定英仙座A中未激波喷出物的质量和物理条件。
• 评估气体温度和团块化对自由-自由吸收所推导出的喷出物质量和发射度的影响。
• 将观测到的射电流量长期衰减与喷出物演化和反向激波动力学模型相协调。
• 改进对超新星遗迹壳层内部冷吸收气体几何结构和电离状态的约束。

提出的方法

• 获取了u-v匹配的LOFAR LBA（30–77 MHz）和VLA L波段（1–2 GHz）图像，具有17"的共同 beam 分辨率。
• 对壳层中每个像素进行发射度以及未吸收前向辐射与吸收后向辐射之比的拟合。
• 将温度和团块化效应纳入自由-自由吸收模型，以评估其对质量估计的影响。
• 汇编历史射电流量测量数据，以建模源在整个射电波段的长期衰减。
• 利用辐射转移原理建模自由-自由吸收，考虑壳层内部冷电离气体的影响。
• 探讨温度和团块化变化如何影响观测到的低频谱行为。

实验结果

研究问题

• RQ1 英仙座A中未激波喷出物的质量是多少？其与假设气体温度和团块化程度的关系如何？
• RQ2 冷吸收气体中低温和高团块化如何影响推导出的发射度和观测到的射电流量？
• RQ3 观测到的射电流量长期衰减是否可由物质被反向激波加热导致的质量损失来解释？
• RQ4 为重现英仙座A中观测到的高比例低频吸收，所需的物理条件（温度、密度、几何结构）是什么？

主要发现

• 在气体温度为100 K时，未激波喷出物质量估计为$2.95 \pm 0.48\,M_\odot$，温度越低，质量值越小。
• 反向激波半径测量为$114 \pm 6$角秒，与激波传播模型一致。
• 仅靠未激波喷出物质量减少，无法解释低频段观测到的长期衰减速率。
• 高吸收要求要么存在极端团块化，要么气体温度低至约10 K，才能使观测结果与低质量喷出物模型相协调。
• 低温与高团块化共同增强自由-自由光学厚度，从而解释了观测到的低频谱转折现象。
• 本研究证明，低频射电观测对冷喷出物的微观物理状态（尤其是团块化和温度）极为敏感。