普遍存在却被遗忘：低质量X射线双星光学光谱中的宽吸收
Ubiquitous yet forgotten: broad absorptions in the optical spectra of low-mass X-ray binaries
AI总结：
研究低质量X射线双星光学光谱中宽吸收现象的普遍存在性和成因，通过光谱数据库分析其特性，发现其与系统亮度反相关，并存在于所有X射线状态中。
摘要：
低质量X射线双星的光学爆发光谱有助于研究极端吸积和喷射现象。尽管一些光谱特征已被详细分析，但光学范围内宽吸收的传统忽视现象在本研究中被首次系统研究。通过六个低质量X射线双星爆发期间的光谱数据库，研究其演变。发现宽吸收普遍存在，多数黑洞低质量X射线双星即使在通常稀少的爆发覆盖下也表现出此特征。其检测不依赖于轨道倾角或致密天体性质，但在轨道周期短于11小时的系统中更常见。它们主要出现在氢巴尔末系列，短波长更强，并存在于所有X射线状态下。发现宽吸收的归一化深度与系统亮度反相关，且在整个样本中线比恒定。基于这些特性，认为宽吸收来自吸积盘稳定、光学厚层，位于 hotter chromosphere-like 区域之下。本研究支持宽吸收在整个爆发过程中持续存在，其可见性受发射线填充宽吸收轮廓和X射线再处理连续谱的遮蔽条件影响。
英文摘要：
Optical outburst spectra of low-mass X-ray binaries enable studies of extreme accretion and ejection phenomena. While some of their spectroscopic features have been analysed in detail, the appearance of broad absorptions in the optical regime has been traditionally neglected. In this work, we introduce the first population study dedicated to these features with the aim to understand their fundamental properties and discuss them in the context of their origin. We complement the study with a spectroscopic database of six low-mass X-ray binaries during outburst, in order to assess their evolution. We find that broad absorptions are ubiquitous, with the majority of black hole low-mass X-ray binaries exhibiting them in spite of a typically scarce outburst coverage. Their detection does not depend on the orbital inclination or the compact object nature, but they seem favoured in systems with orbital periods shorter than < 11 h. They predominantly occur in the hydrogen Balmer series, being stronger at shorter wavelengths, and they are detected across all X-ray states. We find that the normalised depth of these broad absorptions is anti-correlated with the system luminosity, and that they show constant line ratios over the whole sample. Based on these properties, we favour a scenario where BAs arise from a stable, optically thick layer of the accretion disc, below the hotter chromosphere-like region producing the emission line components. Our study is consistent with the continuous presence of broad absorptions during the whole outburst, with their visibility being conditioned by the emission lines filling the broad absorption profile and veiling by the X-ray reprocessed continuum.
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