标题： PSR B1957+20食脉动双星的轨道变化 显示英文标题

标题： Orbital Variability in the Eclipsing Pulsar Binary PSR B1957+20

摘要： 我们对食millisecond脉冲星PSR B1957+20进行了测时观测，将该脉冲星的数据跨度延长至五年以上。 在此期间，该系统的轨道周期变化了大约$\Delta P_b/P_b = 1.6 \times 10^{-7}$，随时间呈二次变化，并显示出轨道周期二阶导数为$\ddot P_b = (1.43 \pm 0.08) \times 10^{-18}\,$秒$^{-1}$。 先前测量到的大负轨道周期导数仅反映了早期观测中的短期行为；现在的轨道周期导数为正且迅速增加。 如果像我们所怀疑的那样，PSR B1957+20系统正在经历类似Algol和RS CVn等其他近距双星中发现的准周期性轨道周期变化，那么PSR B1957+20的$0.025\,M{_\odot}$伴星很可能非退化、对流且磁活动活跃。 显示英文摘要

摘要： We have conducted timing observations of the eclipsing millisecond binary pulsar PSR~B1957+20, extending the span of data on this pulsar to more than five years. During this time the orbital period of the system has varied by roughly $\Delta P_b/P_b = 1.6 \times 10^{-7}$, changing quadratically with time and displaying an orbital period second derivative $\ddot P_b = (1.43 \pm 0.08) \times 10^{-18}\,$s$^{-1}$. The previous measurement of a large negative orbital period derivative reflected only the short-term behavior of the system during the early observations; the orbital period derivative is now positive and increasing rapidly. If, as we suspect, the PSR~B1957+20 system is undergoing quasi-cyclic orbital period variations similar to those found in other close binaries such as Algol and RS CVn, then the $0.025\,M{_\odot}$ companion to PSR~B1957+20 is most likely non-degenerate, convective, and magnetically active.