A fundamental difference between a neutron star (NS) and a black hole (BH) is the absence of a physical surface in the latter. For this reason, any remaining kinetic energy of the matter accreting onto a BH is advected inside its event horizon. In the case of an NS, on the contrary, accreting material is decelerated on the NS surface, and its kinetic energy is eventually radiated away. Copious soft photons produced by the NS surface will affect the properties of the Comptonised component dominating spectra of X-ray binaries in the hard state. Thus, parameters of the Comptonised spectra -- the electron temperature $kT_{\rm e}$ and the Compton $y$-parameter, could serve as an important tool for distinguishing BHs from NSs. In this paper, we systematically analyse heretofore the largest sample of spectra from the BH and NS X-ray binaries in the hard state for this purpose, using archival RXTE/PCA and RXTE/HEXTE observations. We find that the BHs and NSs occupy distinctly different regions in the $y-kT_{\rm e}$ plane with NSs being characterised by systematically lower values of $y$-parameter and electron temperature. Due to the shape of the boundary between BHs and NSs on the $y-kT_{\rm e}$ plane, their one-dimensional $y$ and $kT_{\rm e}$ distributions have some overlap. A cleaner one parameter diagnostic of the nature of the compact object in X-ray binaries is provided by the Compton amplification factor $A$, with the boundary between BHs and NSs lying at $A\approx 3.5-4$. This is by far the most significant detection of the imprint of the event horizon on the X-ray spectra for stable stellar-mass BHs.

中文翻译：

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在 X 射线光谱上观察黑洞事件视界的印记

中子星 (NS) 和黑洞 (BH) 之间的根本区别在于后者没有物理表面。出于这个原因，吸积到 BH 上的物质的任何剩余动能都会在其事件视界内平流。相反，在 NS 的情况下，吸积物质在 NS 表面减速，其动能最终被辐射掉。NS 表面产生的大量软光子将影响在硬态 X 射线双星光谱中占主导地位的 Comptonised 成分的特性。因此，康普顿光谱的参数——电子温度 $kT_{\rm e}$ 和康普顿 $y$-参数，可以作为区分 BHs 和 NSs 的重要工具。在本文中，为此，我们使用档案 RXTE/PCA 和 RXTE/HEXTE 观测系统地分析了迄今为止来自硬态 BH 和 NS X 射线双星的最大光谱样本。我们发现 BHs 和 NSs 在 $y-kT_{\rm e}$ 平面中占据明显不同的区域，NSs 的特征在于 $y$ 参数和电子温度的系统较低值。由于$y-kT_{\rm e}$ 平面上BHs 和NSs 之间边界的形状，它们的一维$y$ 和$kT_{\rm e}$ 分布有一些重叠。康普顿放大因子 $A$ 提供了一个更清晰的参数诊断 X 射线双星中致密物体的性质，BHs 和 NSs 之间的边界位于 $A\大约 3.5-4$。