标题： 4U 1820-30超紧凑低质量X射线双星的轨道星历更新及X射线波段超洪峰调制的探测 显示英文标题

标题： Updated Orbital Ephemeris and Detection of Superhump Modulation in X-ray Band for the Ultra-Compact Low Mass X-ray Binary 4U 1820-30

摘要： 4U 1820-30 是一个超紧凑的低质量 X 射线双星（LMXB），位于球状星团 NGC 6624 的中心附近。 其轨道周期的负导数，从其正弦波状轨道变化的相位演化中观测到，与从理论预测得到的正值相矛盾。 在本文中，我们分析了从 2017 年至 2022 年中期的{\it 中子星内部组成探索者（NICER）}观测获得的光变曲线中的 4U 1820-30 轨道调制。 结合历史记录，从 1976 年至 2002 年的轨道相位演化中测得的轨道导数为$\dot P /P =(-5.21 \pm 0.13) \times 10^{-8}$年$^{-1}$。 没有检测到显著的二阶轨道周期导数，其 2$\sigma$上限为$|\ddot P|<5.48 \times 10^{-22}$秒秒$^{-2}$。 我们讨论了 4U 1820-30 可能的固有轨道周期导数，并认为该双星系统可能具有类似于其他一些 LMXB 的显著质量流出。 此外，还检测到一个周期为$691.6 \pm 0.7$秒的周期性调制，这与在{\it 哈勃空间望远镜}远紫外波段发现的超光度周期一致，该调制也出现在由{\it NICER}收集的X射线光变曲线中。 我们得出结论，这种调制可能是由于类似SU UMa型矮新星和一些LMXBs的吸积盘近日点进动周期为$0.8 \pm 0.1$天引起的。 然而，我们不能排除它可能是由围绕双星系统运行的第三颗分级恒星引起的可能性。 显示英文摘要

摘要： The 4U 1820-30 is a ultra-compact low mass X-ray binary (LMXB) near the center of the globular cluster NGC 6624. Its negative orbital period derivative, observed from the phase evolution of its sinusoidal-like orbital variation, contradicts the positive value obtained from the theoretical prediction. In this paper, we present the analysis of the 4U 1820-30 orbital modulation from light curves obtained from the {\it Neutron star Interior Composition ExploreR (NICER)} observations from 2017 to mid 2022. Combined with historical records, the orbital derivative is measured from the orbital phase evolution between 1976 and 2002 is $\dot P /P =(-5.21 \pm 0.13) \times 10^{-8}$ yr$^{-1}$. No significant second order orbital period derivative is detected with a 2$\sigma$ upper limit of $|\ddot P|<5.48 \times 10^{-22}$ s s$^{-2}$. We discuss the possible intrinsic orbital period derivative of 4U 1820-30 and suggest that this binary system may have a significant mass outflow similar to some other LMXBs. In addition, a periodic modulation with a period of $691.6 \pm 0.7$ s, which is consistent with the superhump period discovered in the far ultraviolet band of the {\it Hubble Space Telescope}, was also detected in in the X-ray light curves collected by {\it NICER}. We conclude that such modulation is probably caused by a period of $0.8 \pm 0.1$ day apsidal precession of accretion disk similar to the SU UMa type dwarf novae and some LMXBs. However we cannot exclude the possibility that it is induced by a hierarchical third star orbiting around the binary system.