标题： M4中的三脉冲星系统PSR B1620-26 显示英文标题

标题： The Triple Pulsar System PSR B1620-26 in M4

摘要： 毫秒脉冲星PSR B1620-26位于球状星团M4中，其伴星是一颗白矮星，轨道周期为半年。脉冲星的明显自转减速率异常大的变化表明存在一个轨道更宽的第二伴星。利用跨越十一年的七百多天的时间观测数据，我们确认了这种异常的时间行为。我们首次明确地证明，由两个不相互作用的开普勒轨道之和组成的时间模型可以解释观测到的信号。允许圆形和椭圆轨道，尽管高度偏心的轨道需要不太可能的轨道几何结构。脉冲星在内轨道中的运动几乎是一个开普勒椭圆，但外伴星的潮汐效应会导致轨道要素的变化。我们测量了轨道投影半长轴的变化，这主要由轨道倾角的变化引起。这一测量结果以及对其他轨道要素变化率的限制，可以显著限制外轨道的特性。我们发现第二伴星的质量约为0.01太阳质量——它肯定低于氢燃烧极限（质量小于0.036太阳质量，95%置信度）——并且目前距离双星系统约35天文单位，轨道周期约为一百年。只有当脉冲星磁场约为3x10^9高斯时，才能允许圆形（或近圆形）轨道，这比典型毫秒脉冲星的磁场强度高一个数量级。在这种情况下，伴星的质量约为1.2x10^-3太阳质量，轨道周期约为62年。 显示英文摘要

摘要： The millisecond pulsar PSR B1620-26, in the globular cluster M4, has a white dwarf companion in a half-year orbit. Anomalously large variations in the pulsar's apparent spin-down rate have suggested the presence of a second companion in a much wider orbit. Using timing observations made on more than seven hundred days spanning eleven years, we confirm this anomalous timing behavior. We explicitly demonstrate, for the first time, that a timing model consisting of the sum of two non-interacting Keplerian orbits can account for the observed signal. Both circular and elliptical orbits are allowed, although highly eccentric orbits require improbable orbital geometries. The motion of the pulsar in the inner orbit is very nearly a Keplerian ellipse, but the tidal effects of the outer companion cause variations in the orbital elements. We have measured the change in the projected semi-major axis of the orbit, which is dominated by precession-driven changes in the orbital inclination. This measurement, along with limits on the rate of change of other orbital elements, can be used to significantly restrict the properties of the outer orbit. We find that the second companion most likely has a mass m~0.01 Msun --- it is almost certainly below the hydrogen burning limit (m<0.036 Msun, 95% confidence) --- and has a current distance from the binary of ~35 AU and orbital period of order one hundred years. Circular (and near-circular) orbits are allowed only if the pulsar magnetic field is ~3x10^9 G, an order of magnitude higher than a typical millisecond pulsar field strength. In this case, the companion has mass m~1.2x10^-3 Msun and orbital period ~62 years.