超紧X射线双星4u 1820 -303负轨道周期导数的环双星盘方案
A Circumbinary Disk Scenario For The Negative Orbital-Period Derivative Of The Ultracompact X-ray Binary 4u1820-303
摘要：
一般认为超紧凑X射线双星是由一颗中子星和一颗氦白矮星施主星组成的。球状星团NGC 6624中的4U 1820-303是最紧凑的双星之一，如果质量转移完全由引力辐射驱动，其轨道周期导数为$\dot{P}/P\sim1.1\times10^{-7}$ ${\rm yr^{-1}}$.然而，最近对来自\textit{Rossi X-ray Timing Explorer}和其他历史记录的16个${\rm yr}$数据的分析得出了过去$35$年的负轨道周期导数。在这项工作中，我们提出了一个进化的circumbinary（CB）磁盘模型来解释这种异常的轨道周期导数。已知4U 1820-30经历了由中子星上失控的热核燃烧引起的超级爆发事件。我们假设，对于一小部分的超爆，喷出的物质的一部分可能会形成一个CB盘周围的二进制。如果这种超暴的重现时间是$\sim10,000$ yr，并且喷出的质量的$\sim10$ %供给CB盘，突然的角动量损失导致暂时的轨道收缩，施主的半径和它的Roche瓣半径不保持同步。在质量转移和角动量损失的驱动下，双星将调整其轨道参数，以恢复一个新的稳定阶段。基于理论分析和数值模拟，我们发现CB盘处所需的馈电质量约为$\sim 10^{-8}$ ${\rm M_{\odot}}$。
英文摘要：
It is generally thought that an ultracompact X-ray binary is composed with a neutron star and a helium white dwarf donor star. As one of the most compact binaries, 4U 1820-303 in globular cluster NGC 6624 was predicted an orbital-period derivative at a rate of $\dot{P}/P\sim1.1\times10^{-7}$ ${\rm yr^{-1}}$ if the mass transfer is fully driven by gravitational radiation. However, the recent analysis of the 16 ${\rm yr}$ data from \textit{Rossi X-ray Timing Explorer} and other historical records yielded a negative orbital-period derivative in the past $35$ yr. In this work, we propose an evolutionary circumbinary (CB) disk model to account for this anomalous orbital-period derivative. 4U 1820-30 was known to undergo superbust events caused by runaway thermal nuclear burning on the neutron star. We assume that for a small fraction of the superbursts, part of the ejected material may form a CB disk around the binary. If the recurrence time of such superbursts is $\sim10,000$ yr and $\sim10$% of the ejected mass feeds a CB disk, the abrupt angular-momentum loss causes a temporary orbital shrink, and the donor's radius and its Roche-lobe radius do not keep in step. Driven by mass transfer and angular-momentum loss, the binary would adjust its orbital parameters to recover a new stable stage. Based on the theoretical analysis and numerical simulation, we find that the required feed mass at the CB disk is approximately $\sim 10^{-8}$ ${\rm M_{\odot}}$.
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