AI 中文总结
该研究利用eFEDS巡天数据，对比变异性选AGN的X射线有/无对应体样本，发现X射线缺失的AGN占比25%，其物理性质与红移分布不同，需纳入黑洞增长的宇宙学估计。
AI 中文摘要
研究目标：表征eROSITA最终赤道深度巡天（eFEDS）中通过变异性选的AGN候选体，利用光学变异性与SRG（Spectrum Roentgen Gamma）/eROSITA的深X射线观测的协同作用，对比通过互补方法选的AGN族群的物理性质。方法：我们使用Arévalo等人通过光学变异性选的AGN候选体样本，将其与X射线选的eFEDS AGN样本交叉匹配；从暗能量光谱仪（DESI）的公开数据产品中获取匹配样本的光谱表征。结果：我们发现分类纯度很高，98%的候选体被证实为宽线AGN/类星体（QSOs）；我们在eFEDS巡天中识别出一个重要族群（25%），它们没有X射线对应体（X-U）。我们发现X射线有对应体（X-D）和X-U样本呈现出不同的物理性质和红移分布：X-U样本在红移z=2处达到峰值，而X-D样本在z=1处达到峰值，近半数z>2.5的变异性选AGN在X射线中未被探测到；这一缺失比例可归因于K校正效应和随光度变化的光谱能量分布（SED）演化，主要导致高吸积率天体在更高红移时从X-D样本中消失；X射线变异性是缺失比例的部分原因，但其与红移的依赖关系尚不明确。整体光谱分析显示，X-D源的光学连续谱比X-U源更陡，宽发射线更强，而禁线和半禁线的等效宽度（EW）在两个样本中相等；最后，宽吸收线（BAL）特征几乎仅存在于X-U族群中。当将X射线选的AGN外推至更广泛的I型AGN族群时，必须考虑这一“缺失”的X射线族群，这可能会影响大质量黑洞增长的宇宙学估计。
英文摘要
(abridged) Aims: To characterize variability-selected AGN candidates within the eROSITA Final Equatorial-Depth Survey (eFEDS), leveraging the synergy between optical variability and deep X-ray observations by SRG (Spectrum Roentgen Gamma)/eROSITA, to compare the physical properties of AGN populations selected through complementary methods. Methods: We used a sample of AGN candidates selected via optical variability by Arévalo et al. and cross-match it to the X-ray selected eFEDS AGN sample. Spectroscopic characterizations are obtained for the matched samples from the Dark Energy Spectroscopic Instrument public data products. Results: We find a high classification purity with 98% candidates confirmed as broad-line AGN/QSOs. We identify a significant population (25%) that lack X-ray counterparts (X-U) in the eFEDS survey. We find that the X-D and X-U samples exhibit distinct physical properties and redshift distributions. The X-U sample peaks at $z = 2$ while the X-D sample at $z =1$, with nearly half of the variability-selected AGN at $z > 2.5$ remaining undetected in X-rays. This missing fraction can be attributed to K-correction effects and SED evolution with luminosity, mainly resulting in high accretion rate objects falling out of the X-D sample at higher redshifts. X-ray variability accounts for a part of the missing fraction but its dependence with redshift is less clear. Ensemble spectral analysis reveals that X-D sources have steeper optical continua and stronger broad emission lines than their X-U counterparts, while the forbidden and semi-forbidden lines have equal EWs in both samples. Finally, broad absorption line (BAL) features are found almost exclusively in the X-U population. This "missing" X-ray population must be accounted for when extrapolating X-ray-selected AGN to the broader Type I population and could impact cosmic estimates of massive black hole growth.
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