利用AstroSat研究黑洞X射线双星Swift J151857-572147中C型QPO的消失
Investigating the disappearance of a type-C QPO in black hole X-ray binary Swift J151857-572147 with AstroSat
AI总结：
利用AstroSat对黑洞X射线双星Swift J151857-572147的观测分析，发现其C型QPO与低频鼓包随光谱态转变同时消失，表明二者均起源于电晕，反映了内吸积几何结构的快速重组。
AI中文摘要：
我们利用AstroSat对黑洞低质量X射线双星Swift J151857.0--572147开展了计时与宽带光谱研究，重点分析其在S1和S2两个时间段的演化。在持续约50.6 ks的第一时段S1中，该源的功率密度谱上存在一个约8.02 Hz的清晰C型QPO和一个低频鼓包（LFB）。到了较晚的时段S2，QPO特征的强度显著降低，不再符合相干QPO的特征，同时LFB也随之消失。能量相关分析显示，S1中QPO的均方根振幅和时间延迟均随能量单调增加，表明该源的倾角较低。QPO和LFB的质心频率在两个轨道段间呈现强正相关，皮尔逊相关系数为0.98。我们的变率分析和硬度-强度图显示，该源的光谱态转变与QPO和LFB的关闭同时发生，宽带光谱参数的演化也支持这一结论。值得注意的是，轨道分辨光谱分析表明，当源从S1过渡到S2时，非热参数、光子指数和电晕的电子温度出现显著偏差，而热参数则呈离散分布，在两个时段间无系统变化。源自热内流的C型QPO和低频宽带噪声成分同时关闭，表明内吸积几何结构发生了快速重组。我们的结果强烈暗示QPO起源于电晕，并证明这两种特征的消失反映了光谱转变过程中康普顿化区域的重大变化。
英文摘要：
We present a timing and broadband spectral study of the AstroSat observation of the black hole low mass X-ray binary Swift J151857.0--572147, focusing on its evolution across two temporal segments, S1 and S2. During the first segment S1 lasting $\sim$ 50.6 ks, the source shows a clear type-C QPO at $\sim8.02$ Hz and a low-frequency bump (LFB) in the power density spectrum. By the later segment S2, the strength of the QPO feature decreases significantly and is no longer consistent with a coherent QPO, along with the simultaneous disappearance of the LFB. An energy-dependent analysis reveals that both the RMS amplitude and the time lag of the QPO in S1 increase monotonically with energy, suggesting a low inclination nature of the source. The QPO and LFB centroid frequencies show a strong positive correlation across orbital segments, with a Pearson coefficient of 0.98. Our variability analysis and hardness-intensity diagram reveal a spectral state transition of the source coincident with the turn-off of the QPO and the LFB, a conclusion supported by the evolution of broadband spectral parameters. Notably, the orbit-resolved spectral analysis reveals that the non-thermal parameters, photon index, and electron temperature of the corona exhibit significant deviation as the source traverses from S1 to S2, whereas the thermal parameters scatter without any systematic variation between the two segments. The simultaneous turn-off of the type-C QPO and the low-frequency broadband noise component, both originating from the hot inner flow, indicates a rapid reorganization of the inner accretion geometry. Our results strongly suggest a coronal origin for the QPO and demonstrate that the disappearance of both features reflects a substantial transformation of the Comptonizing region across the spectral transition.
