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arXiv 2604.04085astro-ph.HE
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重新审视AGN的X射线变异性平面：光子指数的显著作用

Revisiting the X-ray Variability Plane of AGNs: The Significant Role of the Photon Index

University of Science and Technology of China（中国科学技术大学）；Xiamen University（厦门大学）；Huazhong University of Science and Technology（华中科技大学）；Sun Yat-sen University（中山大学）

Ruisong Xia, Hao Liu, Yongquan Xue, Jialai Wang, Guowei Ren, Mouyuan Sun, Shifu Zhu, Mengqiu Huang, Qingwen Wu, Xian-Liang Lu, Zhen-Bo Su, Shuying Zhou

备注：
13 pages,3 figures,accepted for publication in ApJ
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AI总结：

研究通过分析112个AGN的X射线变异性，发现光子指数对变异性时间尺度有显著影响，修正了变异性平面模型，得出最佳拟合关系。

摘要：

X射线变异性为研究活动星系核（AGNs）最内区域提供了有力探测手段，揭示了吸积过程和火球结构。先前研究建立了X射线变异性时间尺度、黑洞质量与亮度之间的相关性，形成AGN X射线变异性平面。早期研究注意到X射线能谱光子指数与X射线变异性之间可能存在联系，但很少被纳入后续变异性平面分析。此外，早期研究中的样本量有限，限制了变异性平面的稳健性和普遍性。本文通过4XMM-DR14目录中的112个AGN和399次曝光数据，利用新发展的拟合方法BADDAT，探讨了X射线变异性时间尺度、黑洞质量、亮度和光子指数之间的相关性。分析证实了静止框架变异性时间尺度（τ_rest）与黑洞质量（M_BH）之间的依赖关系，并将光子指数（Γ）纳入变异性平面，得出最佳拟合关系：log(τ_rest/s) = 1.22 log(M_BH/M_⊙) - 0.24Γ - 3.53，该关系优于仅包含M_BH的模型。相比之下，亮度的加入并未产生同等改进。与Γ的相关性可能反映了Compton化和火球几何结构的影响。

英文摘要：

X-ray variability provides a powerful probe of the innermost regions of active galactic nuclei (AGNs), offering valuable insights into the accretion process and the structure of the corona. Previous studies have established a correlation between the X-ray variability timescale, black hole mass, and luminosity, forming the AGN X-ray variability plane. A possible link between the X-ray spectral photon index and X-ray variability was noted in early studies but has rarely been incorporated into subsequent analyses of the variability plane. Moreover, the limited sample sizes in earlier works have limited the robustness and universality of the X-ray variability plane. In this work, we compile a sample of 112 AGNs with 399 exposures from the 4XMM-DR14 catalog and constrain the correlations between X-ray variability timescale, black hole mass, luminosity, and photon index using the recently developed fitting method, BADDAT {(Baseline-Aware Dependence fitting for DAmping Timescales)}, which enables a robust exploration of an extended parameter space. Our analysis confirms the dependence of the rest-frame variability timescale ($τ_{\rm rest}$) on black hole mass ($M_{\rm BH}$) and further incorporates the photon index ($Γ$) into the variability plane, yielding a best-fit relation of $\log (τ_{\rm rest}/{\rm s}) = 1.22\log (M_{\rm BH}/M_\odot) - 0.24Γ- 3.53$, which is strongly favored over the model with $M_{\rm BH}$ alone. In contrast, the inclusion of luminosity does not produce a comparable improvement. The correlation with $Γ$ likely reflects the effects of Comptonization and the geometry of the corona.

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