发表机构
Università degli Studi di Cagliari; INAF – IASF Palermo; Politecnico di Milano; INAF – Osservatorio Astronomico di Roma; INAF – Osservatorio Astronomico di Brera; Università degli Studi di Palermo; INAF-Osservatorio Astrofisico di Torino（卡利亚里大学; 意大利国家天体物理研究所巴勒莫天文物理研究所; 米兰理工大学; 意大利国家天体物理研究所罗马天文台; 意大利国家天体物理研究所布里埃拉天文台; 巴勒莫大学; 意大利国家天体物理都灵天文台）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究通过XMM-Newton观测，在ULX候选体NGC 7456 ULX-1中探测到千秒级硬X射线延迟，并利用光谱与互相关分析揭示其源于超爱丁顿吸积流中涨落传播与光子扩散，排除了中等质量黑洞吸积体。
AI 中文摘要
背景。超亮X射线源（ULXs）在许多情况下被认为是由致密天体上的超爱丁顿吸积提供能量。虽然在多个ULX中已探测到软X射线延迟，但硬X射线延迟仍然罕见且了解甚少。目标。我们研究NGC 7456 ULX-1中X射线延迟的时间及能量依赖性，以限制其物理起源并探测超爱丁顿吸积流。方法。我们分析了2018年和2023年获取的两组最深的XMM-Newton观测数据。使用针对泊松低计数数据优化的自适应分箱技术，对硬（1-10 keV）和软（0.3-1 keV）光变曲线进行互相关分析。我们在连续的10千秒间隔内测量延迟，并研究其能量依赖性。光谱使用热模型和康普顿化模型进行拟合。结果。我们在两次观测中均探测到显著的硬X射线延迟，在快速通量变化阶段，硬发射延迟约10^3秒。这些延迟主要由最低能量光子驱动。光谱建模表明，吸积流以康普顿化为主，包含一个较冷的、扩展的外层区域和一个较热的、紧凑的内流，内流嵌入在光学厚的风之中。我们将这些延迟解释为吸积率涨落向内传播与密集外流中光子扩散的综合效应。涨落首先增强软发射的外层区域，然后向内传播至较热的内流，在那里光子在逃逸前经历更强的康普顿化，并产生千秒级延迟。推断出的较小内发射半径不利于中等质量黑洞吸积体。结论。延迟的符号、幅度和能量依赖性不利于标准混响机制。在光学厚风中，传播驱动的变率与辐射转移相结合，似乎在塑造超爱丁顿吸积流的时间特性方面发挥着重要作用。
英文摘要
Context. Ultraluminous X-ray sources (ULXs) are thought to be powered, in many cases, by super-Eddington accretion onto compact objects. While soft X-ray lags have been detected in several ULXs, hard lags remain rare and poorly understood. Aims. We investigate the temporal and energy dependence of X-ray lags in NGC 7456 ULX-1 to constrain their physical origin and probe the super-Eddington accretion flow. Methods. We analyzed the two deepest XMM-Newton observations, taken in 2018 and 2023. Hard (1-10 keV) and soft (0.3-1 keV) light curves were cross-correlated using adaptive-binning techniques optimized for Poissonian low-count data. We measured lags over consecutive 10 ks intervals and investigated their energy dependence. Spectra were modeled using thermal and Comptonization models. Results. We detect significant hard X-ray lags in both observations, with the hard emission delayed by $\sim 10^3$ s during phases of rapid flux variability. The delays are primarily driven by the lowest-energy photons. Spectral modeling indicates a Comptonization-dominated flow comprising a cooler, extended outer region and a hotter, compact inner flow embedded in an optically thick wind. We interpret the delays as the combined effect of inward propagation of accretion-rate fluctuations and photon diffusion within the dense outflow. Fluctuations first enhance the soft-emitting outer regions and then propagate toward the hotter inner flow, where photons undergo stronger Comptonization before escaping with a kilosecond delay. The small inferred inner emitting radius disfavors an intermediate-mass black hole accretor. Conclusions. The sign, amplitude, and energy dependence of the delays disfavor standard reverberation. Propagation-driven variability coupled with radiative transfer in optically thick winds appears to play a major role in shaping the timing properties of super-Eddington accretion flows.
CommentsAccepted for publication, A&A, on 29/09/2026
