[论文解读] Obscuration of Supersoft X-ray Sources by Circumbinary Material - A Way to Hide Type Ia Supernova Progenitors?

Mikkel Nielsen, C. Dominik+2 more|UvA-DARE (University of Amsterdam)|Jul 26, 2012

Gamma-ray bursts and supernovae参考文献 51被引用 11

一句话总结

本文提出，在单简并型Ia超新星前身体系统中，环绕双星的物质可遮蔽吸积白矮星发出的超软X射线辐射，从而解释观测到的此类源数量不足的现象。通过辐射转移模型，研究显示在约1 AU的间距下，质量损失率在$10^{-9}$–$10^{-8}\,M_\odot$ yr$^{-1}$的范围内，可完全遮蔽X射线辐射，使这些系统在钱德拉望远镜观测中不可见，可能隐藏了相当大一部分前身体。

ABSTRACT

The progenitors of supernovae (SNe) type Ia are usually assumed to be either a single white dwarf (WD) accreting from a non-degenerate companion (the SD channel) or the result of two merging WDs (DD channel). However, no consensus currently exists as to which progenitor scenario is the correct one, or whether the observed SN Ia rate is produced by a combination of both channels. Unlike a DD progenitor a SD progenitor is expected to emit supersoft X-rays for a prolonged period of time (~1 Myr) as a result of the burning of accreted matter on the surface of the WD. An argument against the SD channel as a significant producer of SNe type Ia has been the lack of observed supersoft X-ray sources (SSS) and the lower-than-expected integrated soft X-ray flux from elliptical galaxies. We wish to determine if it is possible to obscure the supersoft X-ray emission from a nuclear burning white dwarf in an accreting single degenerate binary system. In case of obscured systems we wish to determine their general observational characteristics. We examine the emergent X-ray emission from a canonical SSS system surrounded by a spherically symmetric configuration of material, assuming a black body spectrum with T_BB=50 eV and L=10^38 erg/s. The circumbinary material is assumed to be of solar chemical abundances, and we leave the mechanism behind the mass loss into the circumbinary region unspecified. If steadily accreting, nuclear burning WDs are canonical SSS our analysis suggests that they can be obscured by relatively modest circumbinary mass loss rates. This may explain the discrepancy of SSS compared to the SN Ia rate inferred from observations if the SD progenitor scenario contributes significantly to the SN Ia rate. Recycled emissions from obscured systems may be visible in other wavebands than X-rays. It may also explain the lack of observed SSS in symbiotic binary systems.

研究动机与目标

• 调查环绕双星的物质是否可遮蔽单简并型Ia超新星前身体中吸积白矮星的超软X射线辐射。
• 确定实现显著X射线遮蔽所需的物质损失率与双星间距。
• 解释预期的超软X射线源数量与观测中偏低计数之间的差异，特别是在椭圆星系中。
• 探索被遮蔽系统在其他波段（如H$\alpha$、红外或射电）中的潜在观测特征。
• 评估在共生双星系统中未观测到超软源是否可能源于内在遮蔽。

提出的方法

• 建立一个典型的超软X射线源模型，其光谱为黑体谱，温度$T_{\text{bb}} = 50$ eV，光度$L = 10^{38}$ erg s$^{-1}$。
• 假设环绕双星的物质呈球对称分布，具有太阳丰度，质量损失机制未明确指定。
• 利用辐射转移计算确定包周介质中的电离结构及X射线光学深度。
• 计算不同双星间距（$a \sim 1$ AU至$100$ AU）和质量损失率（$\dot{M} = 10^{-9}$–$10^{-6}\,M_\odot$ yr$^{-1}$）下的出射X射线谱。
• 识别形成清晰电离区的临界质量损失率，表明可能的遮蔽效应。
• 评估在钱德拉望远镜中的可探测性，并预测被遮蔽系统在H$\alpha$、红外和射电波段的可观测特征。

实验结果

研究问题

• RQ1 在单简并型前身体系统中，环绕双星的物质是否可遮蔽稳定燃烧白矮星的超软X射线辐射？
• RQ2 需要多高的质量损失率与多大的双星间距，才能使此类系统在X射线波段不可见？
• RQ3 尽管预期Ia超新星发生率很高，为何观测到的超软X射线源如此稀少？
• RQ4 被遮蔽的系统是否可能在H$\alpha$或射电波段被探测到？
• RQ5 来自伴星的内在质量损失在多大程度上可解释共生双星系统中未观测到超软源的现象？

主要发现

• 在约1 AU的双星间距下，质量损失率在$10^{-9}$–$10^{-8}\,M_\odot$ yr$^{-1}$的范围内，足以显著遮蔽典型超软X射线源的X射线辐射。
• 对于$ a \sim 1$ AU的系统，此类遮蔽可使源在钱德拉望远镜中完全不可见，即使质量损失率适中亦然。
• 形成清晰电离区的临界质量损失率远高于实现完全X射线遮蔽所需的量级，高若干数量级。
• 即使在$\sim 100$ AU的间距下，质量损失率$\sim 10^{-7}\,M_\odot$ yr$^{-1}$也足以实现显著遮蔽，尽管此类宽距系统可能难以维持稳定燃烧。
• 被遮蔽的系统可能在H$\alpha$、红外或射电波段以复合辐射云的形式被探测到，为探测提供了替代路径。
• 质量损失率低至$10^{-11}$–$10^{-10}\,M_\odot$ yr$^{-1}$的稳定燃烧白矮星可能在X射线波段完全不可探测，暗示存在大量隐藏的前身体。