标题： 通过频率分辨光谱探测黑洞双星MAXI J1820+070的吸积几何结构 显示英文标题

标题： Accretion geometry of the black hole binary MAXI J1820+070 probed by frequency-resolved spectroscopy

摘要： 硬态和硬中间态下X射线双星内吸积流的几何结构仍存在争议。利用MAXI J1820+070（一个黑洞X射线双星）在2018年爆发上升阶段的NICER观测数据，我们研究了时变特性（特别是显著铁K$\alpha$线的特征变率频率）的演化。通过频率分辨光谱分析，我们发现反射发生在明亮硬态期间远离康普顿化区域的大距离处。在硬态到软态的转变过程中，变率特性表明反射面靠近X射线源移动。同时，铁线峰值从6.5 keV移至~7 keV，与接近黑洞的高倾角盘所预期的一致。此外，我们还发现，在源变软的过程中，归一化均方根（rms）变率对能量和傅里叶频率的依赖关系发生了显著变化。归一化均方根随能量的变化、铁线的线轮廓以及由频率分辨光谱追踪到的时变特性共同支持截断盘/内流几何结构的图像。 显示英文摘要

摘要： The geometry of the inner accretion flow in the hard and hard-intermediate states of X-ray binaries remains controversial. Using NICER observations of the black hole X-ray binary MAXI J1820+070 during the rising phase of its 2018 outburst, we study the evolution of the timing properties, in particular the characteristic variability frequencies of the prominent iron K$\alpha$ line. Using frequency-resolved spectroscopy, we find that reflection occurs at large distances from the Comptonizing region in the bright hard state. During the hard- to soft transition, the variability properties suggest the reflector moves closer to the X-ray source. In parallel, the peak of the iron line shifts from 6.5 to ~7 keV, becoming consistent with that expected of from a highly inclined disc extending close to the black hole. We additionally find significant changes in the dependence of the root-mean-square (rms) variability on both energy and Fourier frequency as the source softens. The evolution of the rms-energy dependence, the line profile, and the timing properties of the iron line as traced by the frequency-resolved spectroscopy all support the picture of a truncated disc/inner flow geometry.