发表机构
Institut für Astronomie und Astrophysik（天文与天体物理研究所）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
利用Swift/XRT长期监测数据，通过硬度比和穿越指数对29个超亮X射线源进行分类，识别出软波段、硬波段和混合波段三类，并探讨其与吸积天体类型及长期调制的关联。
AI 中文摘要
我们利用超过3500次存档的Swift/XRT观测，刻画了29个超亮X射线源（ULX）的长期硬度行为。我们根据硬度比（HR；1.5--10/0.3--1.5 keV计数率）及其置信界限，将观测分为软波段主导、硬波段主导或不确定三类。我们推导出$f_{\ m soft}$和$f_{\ m hard}$，并将穿越指数（CI）定义为在$HR=1$两侧连续且被可靠分类的观测所占的比例。我们使用基于规则的分类和无监督层次聚类分析这些描述符。基于规则的分析识别出8个软波段主导、5个硬波段主导和16个混合波段系统；聚类分析得出相似的三组结构。已知或候选的吸积天体类型倾向于占据波段主导组，而未知吸积天体在混合波段组中更为常见。所有四个在轨道或超轨道背景下报告了长期X射线调制的源均被分类为混合波段。在29次留一法试验中，有25次倾向于三簇解，而自举检验发现波段主导组比混合波段组更稳定。由于分类使用观测计数率，这些组代表Swift/XRT特征而非内在光谱状态。响应折叠测试表明，建模的HR依赖于吸收柱密度和光子指数。对一个代表性的低计数源进行贝叶斯重新分析，将其CI从56.3%降至38.5%，同时保持其混合波段分类。随机降采样不会产生CI的系统性偏移；将四个源的观测减半，其中位CI变化最多约10%。更大样本及独立的吸收约束可以检验这些观测类别、吸积天体类型、长期周期性调制及其他X射线性质之间的联系。
英文摘要
We characterize long-term hardness behavior in 29 ultraluminous X-ray sources (ULXs) using over 3500 archival Swift/XRT observations. We classify observations as soft-band dominated, hard-band dominated, or uncertain from the hardness ratio (HR; 1.5--10/0.3--1.5 keV count rates) and its confidence bounds. We derive $f_{\rm soft}$ and $f_{\rm hard}$ and define the Crossing Index (CI) as the fraction of consecutive confidently classified observations on opposite sides of $HR=1$. We analyze these descriptors with rule-based classification and unsupervised hierarchical clustering. The rule-based analysis identifies eight soft-band-dominated, five hard-band-dominated, and 16 mixed-band systems; clustering yields a similar three-group structure. Known or candidate accretor types tend to occupy band-dominated groups, whereas unknown accretors are more common in the mixed-band group. All four sources with reported long-term X-ray modulation discussed in an orbital or super-orbital context are classified as mixed-band. The three-cluster solution is preferred in 25 of 29 leave-one-out trials, while bootstrap tests find the band-dominated groups more stable than the mixed-band group. As the classification uses observed count rates, the groups represent Swift/XRT signatures rather than intrinsic spectral states. Response-folding tests show that modeled HR depends on absorption column density and photon index. Bayesian reanalysis of a representative low-count source reduces CI from 56.3\% to 38.5\% while preserving its mixed-band classification. Random downsampling produces no systematic shift in CI; halving the observations changes median CI by at most $\sim10\%$ across four sources. Larger samples with independent absorption constraints can test links among these observational classes, accretor type, long-term periodic modulation, and other X-ray properties.
CommentsAccepted for publication in Astronomy & Astrophysics
DOI：10.1051/0004-6361/202662175
