[论文解读] Discovery of a recurrent spectral evolutionary cycle in the ultra-luminous X-ray sources Holmberg II X-1 and NGC 5204 X-1

A. Gúrpide, O. Godet+3 more|arXiv (Cornell University)|Jun 10, 2021

Astrophysical Phenomena and Observations参考文献 5被引用 2

一句话总结

本研究首次提供了在两个超亮X射线源（ULXs）——Holmberg II X-1 和 NGC 5204 X-1——中存在周期性光谱演化循环的稳健证据，该循环以硬超亮（HUL）、软超亮（SUL）和超软超亮（SSUL）状态之间的转换为特征。该循环由质量吸积率的周期性增加驱动，导致吸积喷管变窄，而随机风变异性则引起快速的SUL至SSUL状态转换，NGC 5204 X-1中观测到的200天周期性可能暗示存在超轨道调制。

ABSTRACT

Most ultra-luminous X-ray sources (ULXs) are now thought to be powered by stellar-mass compact objects accreting at super-Eddington rates. While the discovery of evolutionary cycles have marked a breakthrough in our understanding of the accretion flow changes in the sub-Eddington regime in Galactic Black Hole Binaries, their evidence in the super-Eddington regime remained elusive. However, recent circumstantial evidence had hinted the presence of a recurrent evolutionary cycle in two archetypal ULXs: Holmberg II X-1 and NGC 5204 X-1. Here we build on our previous work and exploit the long-term high-cadence monitoring of Swift-XRT in order to provide evidence of the evolutionary cycle in these two sources and investigate the main physical parameters inducing their spectral transitions. We study the long-term evolution of both sources using hardness-intensity diagrams (HID) and by means of Lomb-Scargle periodograms and Gaussian processes modelling to look for periodic variability. We show that both sources follow a recurrent evolutionary pattern in the HID that can be characterized by the hard ultraluminous (HUL) and soft ultraluminous (SUL) spectral regimes, and a third state with characteristics similar to the supersoft ultraluminous (SSUL) state. The transitions between the soft states seem aperiodic, as revealed by timing analysis of the light curve of Holmberg II X-1, albeit further investigation is warranted. The light curve of NGC 5204 X-1 shows a periodicity of $\sim$ 200 days, possibly associated with the duration of the evolutionary cycle. We support a scenario in which the spectral changes from HUL to SUL are due to a periodic increase of the mass-transfer rate and subsequent narrowing of the opening angle of the supercritical funnel. The narrower funnel, combined with stochastic variability imprinted by the wind, might explain the SUL--SSUL spectral changes.

研究动机与目标

• 为超爱丁顿吸积环境下此前难以捉摸的ULX中周期性光谱演化循环提供稳健证据。
• 研究ULX中HUL、SUL和SSUL状态之间光谱转换的物理机制。
• 确定Holmberg II X-1和NGC 5204 X-1光曲线中的周期性变化是否与光谱循环相关。
• 检验吸积率增加导致喷管变窄可解释HUL至SUL状态转换的假设。
• 探讨NGC 5204 X-1中200天周期性的起源及其与轨道或超轨道调制的潜在关联。

提出的方法

• 利用Swift-XRT进行长期高时间分辨率监测，以追踪通量和光谱随时间的演化。
• 构建硬度-强度图（HID）以可视化光谱状态转换。
• 应用Lomb-Scargle周期图和高斯过程建模，以检测光曲线中的周期性和非周期性变异性。
• 使用Chandra、XMM-Newton、NuSTAR和Swift-XRT的多epoch X射线数据，结合物理解释的光谱模型进行拟合，以表征每种状态。
• 分析Holmberg II X-1光曲线的时域特性，评估SUL和SSUL状态下的变异性模式。
• 比较两个ULX的光谱和时域行为，以推断共同的物理机制。

实验结果

研究问题

• RQ1 Holmberg II X-1和NGC 5204 X-1是否在HUL、SUL和SSUL状态之间表现出可重复的光谱演化循环？
• RQ2 HUL至SUL状态转换的物理机制是什么？该过程是否具有周期性？
• RQ3 SUL和SSUL状态中观测到的快速非周期性光谱变异性由何引起？
• RQ4 NGC 5204 X-1中的200天周期性是否与完整的光谱循环相关？其起源是什么？
• RQ5 两个ULX之间的相似性是否支持相同的吸积流几何结构或吸积体类型？

主要发现

• Holmberg II X-1和NGC 5204 X-1在硬度-强度图中均表现出清晰且可重复的光谱演化循环，依次经历HUL、SUL和SSUL状态。
• HUL至SUL的过渡与质量吸积率的周期性增加一致，导致喷管变窄，从而减少视线方向的遮挡。
• Holmberg II X-1的SUL状态中快速的非周期性变异性可归因于随机风团块穿过视线方向，由于喷管变窄而加剧。
• NGC 5204 X-1的光曲线显示出约200天的稳定周期性，可能对应于光谱循环的完整时长。
• SSUL至HUL状态转换的缺失可通过该模型自然解释：一旦喷管完全闭合，视线仅能观测到风光球层，抑制了短时标变异性，从而稳定了SSUL状态。
• NGC 5204 X-1中的200天周期性可能为超轨道周期，暗示轨道周期约为10天，与超巨星伴星充满其洛希瓣的模型一致。