The existence of a ‘density cusp’—a localized increase in number—of stellar-mass black holes near a supermassive black hole is a fundamental prediction of galactic stellar dynamics. The best place to detect such a cusp is in the Galactic Centre, where the nearest supermassive black hole, Sagittarius A*, resides. As many as 20,000 black holes are predicted to settle into the central parsec of the Galaxy as a result of dynamical friction; however, so far no density cusp of black holes has been detected. Low-mass X-ray binary systems that contain a stellar-mass black hole are natural tracers of isolated black holes. Here we report observations of a dozen quiescent X-ray binaries in a density cusp within one parsec of Sagittarius A*. The lower-energy emission spectra that we observed in these binaries is distinct from the higher-energy spectra associated with the population of accreting white dwarfs that dominates the central eight parsecs of the Galaxy. The properties of these X-ray binaries, in particular their spatial distribution and luminosity function, suggest the existence of hundreds of binary systems in the central parsec of the Galaxy and many more isolated black holes. We cannot rule out a contribution to the observed emission from a population (of up to about one-half the number of X-ray binaries) of rotationally powered, millisecond pulsars. The spatial distribution of the binary systems is a relic of their formation history, either in the stellar disk around Sagittarius A* (ref. 7) or through in-fall from globular clusters, and constrains the number density of sources in the modelling of gravitational waves from massive stellar remnants, such as neutron stars and black holes.

中文翻译：

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银河系中央秒差距中静止 X 射线双星的密度尖顶

超大质量黑洞附近恒星质量黑洞的“密度尖峰”（数量的局部增加）的存在是对银河系恒星动力学的基本预测。探测这种尖点的最佳地点是银河系中心，最近的超大质量黑洞人马座 A* 就在那里。由于动力摩擦，预计多达 20,000 个黑洞将落入银河系的中央秒差距；然而，到目前为止，还没有探测到黑洞的密度尖峰。包含恒星质量黑洞的低质量 X 射线双星系统是孤立黑洞的天然示踪剂。在这里，我们报告了对人马座 A* 1 秒差距内密度尖点中的十几个静止 X 射线双星的观察结果。我们在这些双星中观察到的低能发射光谱不同于与占据银河系中央 8 秒差距的吸积白矮星种群相关的高能光谱。这些 X 射线双星的特性，特别是它们的空间分布和光度函数，表明在银河系的中央秒差距中存在数百个双星系统和更多孤立的黑洞。我们不能排除对观察到的来自旋转驱动的毫秒脉冲星（最多约为 X 射线双星数量的二分之一）的发射的贡献。双星系统的空间分布是它们形成历史的遗迹，无论是在人马座 A* 周围的星盘中（参考文献 7），还是通过球状星团的坠落，