The High Energy X-ray Probe (HEX-P): Probing Accretion onto Stellar Mass Black Holes

Accretion is a universal astrophysical process that plays a key role in cosmic history, from the epoch of reionization to galaxy and stellar formation and evolution. Accreting stellar-mass black holes in X-ray binaries are one of the best laboratories to study the accretion process and probe strong gravity---and most importantly, to measure the angular momentum, or spin, of black holes, and its role as a powering mechanism for relativistic astrophysical phenomena. Comprehensive characterization of the disk-corona system of accreting black holes, and their co-evolution, is fundamental to measurements of black hole spin. Here, we use simulated data to demonstrate how key unanswered questions in the study of accreting stellar-mass black holes will be addressed by the High Energy X-ray Probe (HEX-P). HEX-P is a probe-class mission concept that will combine high spatial resolution X-ray imaging and broad spectral coverage (0.2-80 keV) with a sensitivity superior to current facilities (including XMM-Newton and NuSTAR) to enable revolutionary new insights into a variety of important astrophysical problems. We illustrate the capability of HEX-P to: 1) measure the evolving structures of black hole binary accretion flows down to low (<0.1%) Eddington-scaled luminosities via detailed X-ray reflection spectroscopy; 2) provide unprecedented spectral observations of the coronal plasma, probing its elusive geometry and energetics; 3) perform detailed broadband studies of stellar mass black holes in nearby galaxies, thus expanding the repertoire of sources we can use to study accretion physics and determine the fundamental nature of black holes; and 4) act as a complementary observatory to a range of future ground and space-based astronomical observatories, thus providing key spectral measurements of the multi-component emission from the inner accretion flows of black hole X-ray binaries.吸积（Accretion）是一种普适性天体物理过程，在宇宙历史中扮演关键角色，其影响覆盖从宇宙再电离纪元直至星系与恒星的形成与演化全过程。X射线双星中的吸积恒星质量黑洞，是研究吸积过程、探测强引力场的绝佳实验平台之一；尤为重要的是，借此可测量黑洞的角动量（即自旋），并探究其作为相对论天体物理现象供能机制的作用。对吸积黑洞的盘冕系统及其协同演化进行全面表征，是精确测量黑洞自旋的核心基础。本文依托模拟数据，演示了高能X射线探测器（High Energy X-ray Probe，以下简称HEX-P）将如何解决吸积恒星质量黑洞研究领域中若干关键悬而未决的问题。HEX-P属于探测器级任务概念，其将结合高空间分辨率X射线成像与宽光谱覆盖范围（0.2~80 keV），且灵敏度优于现有观测设备（包括XMM-Newton与NuSTAR），从而为诸多重要天体物理问题带来革命性的全新认知。我们将展示HEX-P具备如下能力：1. 通过精细的X射线反射光谱学，在低光度（<0.1%爱丁顿标度光度）条件下解析黑洞双星吸积流的演化结构；2. 获取前所未有的冕层等离子体光谱观测数据，探究其难以捉摸的几何形态与能量学特性；3. 对邻近星系中的恒星质量黑洞开展细致的宽波段研究，从而扩充可用于研究吸积物理、解析黑洞本质的源样本库；4. 作为互补观测台站，配合未来一系列地基与天基天文观测设备，对黑洞X射线双星内区吸积流的多组分辐射开展关键光谱测量。