超致密低质量X射线双星4U 1820-30的更新轨道星历及X射线波段超驼峰调制的探测
Updated Orbital Ephemeris and Detection of Superhump Modulation in X-ray Band for the Ultra-Compact Low Mass X-ray Binary 4U 1820-30
AI总结：
本文利用NICER观测更新了超致密低质量X射线双星4U 1820-30的轨道星历，测得负轨道周期导数并推测存在显著质量外流，同时在X射线波段首次探测到与远紫外超驼峰一致的周期性调制，提出其可能源于吸积盘拱线进动或第三颗伴星。
摘要：
4U 1820-30是位于球状星团NGC 6624中心附近的超致密低质量X射线双星（LMXB）。从其类正弦轨道变化相位演化观测到的负轨道周期导数，与理论预测得到的正值相矛盾。本文利用中子星内部组成探测器（NICER）2017年至2022年中期的观测光变曲线，对4U 1820-30的轨道调制进行了分析。结合历史记录，从1976年至2002年间的轨道相位演化测得轨道导数为$\dot P /P =(-5.21 \pm 0.13) \times 10^{-8}$ yr$^{-1}$。未检测到显著的二阶轨道周期导数，其2σ上限为$|\ddot P|<5.48 \times 10^{-22}$ s s$^{-2}$。我们讨论了4U 1820-30可能的内禀轨道周期导数，并提出该双星系统可能具有与其他一些LMXB类似的显著质量外流。此外，在NICER收集的X射线光变曲线中还检测到一个周期为$691.6 \pm 0.7$ s的周期性调制，该周期与哈勃空间望远镜在远紫外波段发现的超驼峰周期一致。我们推断这种调制很可能由周期为$0.8 \pm 0.1$天的吸积盘拱线进动引起，类似于SU UMa型矮新星和一些LMXB。然而，我们不能排除它是由围绕该双星系统运行的层级第三颗恒星所诱发的可能性。
英文摘要：
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$σ$ 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.
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