标题： 揭示超紧致低质量X射线双星4U 1820-30的反射光谱 显示英文标题

标题： Unveiling the reflection spectrum in the ultracompact LMXB 4U 1820-30

摘要： 4U 1820-30 是位于球状星团 NGC 6624 中的一个超紧致X射线双星系统，由一颗从氦白矮伴星吸积物质的中子星组成，该类型恒星已知最短轨道周期为11.4分钟。 尽管进行了广泛的研究，但相对论铁K发射线的探测报告不一致，且尚未实现系统的倾角测量。 在这项工作中，我们研究了4U 1820-30的宽带能谱和偏振特性，探讨反射分量的存在及其在塑造偏振信号中的作用。 我们分析了来自NICER、NuSTAR和IXPE的同步X射线观测数据。 连续谱用一个盘面黑体辐射、幂律以及种子光子起源于边界层的康普顿化组分来建模。 我们检测到了一个强烈的反射分量，并首次使用两个自洽模型（{\tt Relxillns}和{\tt Rfxconv}）对其进行描述，从而提供了系统倾角（约31度）的测量值，支持低倾角假设。 在吸积盘和星际介质中检测到亚太阳丰度的铁，可能与源位于贫金属球状星团的位置有关。 偏振在2-4 keV能量范围内的上限为$1.2\%$，在7-8 keV范围内增加到约$8\%$。 盘面预计正交于这些成分，这可能有助于解释低能区观测到的偏振减少现象。 然而，我们发现的高偏振程度挑战了现有的模型，同时也考虑到从光谱分析得出的相对较低倾角。 显示英文摘要

摘要： 4U 1820-30 is a ultracompact X-ray binary located in the globular cluster NGC 6624, consisting of a neutron star accreting material from a helium white dwarf companion characterized by the shortest known orbital period for this type of star (11.4 minutes). Despite extensive studies, the detection of the relativistic Fe K emission line, has been inconsistently reported and no measurement of the system inclination has been achieved. In this work, we investigate the broadband spectral and polarimetric properties of 4U 1820-30, exploring the presence of a reflection component and its role in shaping the polarization signal. We analyzed simultaneous X-ray observations from NICER, NuSTAR, and IXPE. The spectral continuum was modeled with a disk blackbody, a power law, and a Comptonization component with seed photons originating from a boundary layer. We detected a strong reflection component, described, for the first time, with two self-consistent models ({\tt Relxillns} and {\tt Rfxconv}), allowing us to provide a measurement of the system inclination angle (about 31 degrees), supporting the low-inclination hypothesis. Subsolar iron abundances were detected in the accretion disk and interstellar medium, probably related to the source location in a metal-poor globular cluster. The polarization increases from an upper limit of $1.2\%$ in the 2--4 keV band up to about $8\%$ in the 7--8 keV range. The disk is expected to be orthogonally polarized to these components, which may help to explain the decreasing of the observed polarization at low energies. However, the high polarization degree we found challenges the current models, also taking into consideration the relatively low inclination angle derived from the spectral analysis.