The recent discovery of the first known accretion-powered millisecond pulsar with the Rossi X-Ray Timing Explorer provides the first direct probe of the interaction of an accretion disk with the magnetic field of a weakly magnetic (B 1010 G) neutron star. We demonstrate that the presence of coherent pulsations from a weakly magnetic neutron star over a wide range of accretion rates places strong constraints on models of the disk-magnetosphere interaction. We argue that the simple 3/7 scaling law for the Keplerian frequency at the magnetic interaction radius, widely used to model disk accretion onto magnetic stars, is not consistent with observations of SAX J1808.4-3658 for most proposed equations of state for stable neutron stars. We show that the usually neglected effects of multipole magnetic moments, radiation drag forces, and general relativity must be considered when modeling such weakly magnetic systems. Using only very general assumptions, we obtain a robust estimate of μ (1-10) × 1026 G cm3 for the dipole magnetic moment of SAX J1808.4-3658, implying a surface dipole field of ~108-109 G at the stellar equator. We therefore infer that after the end of its accretion phase, this source will become a normal millisecond radio pulsar. Finally, we compare the physical properties of this pulsar with those of the nonpulsing, weakly magnetic neutron stars in low-mass X-ray binaries and argue that the absence of coherent pulsations from the latter does not necessarily imply that these neutron stars have significantly different magnetic field strengths from SAX J1808.4-3658.
展开▼
机译：最近发现的第一个已知的具有罗西X射线定时资源管理器的增力动力毫秒脉冲星，提供了第一个直接探测吸积盘与弱磁（B 1010 G）中子星磁场的相互作用的方法。我们证明了在较宽的吸积率范围内，弱磁中子星的相干脉动的存在对磁盘-磁层相互作用模型提出了严格的约束。我们认为，广泛用于对磁星上的圆盘吸积进行建模的，在磁相互作用半径处的开普勒频率的简单3/7标度定律与大多数提出的稳定状态方程有关SAX J1808.4-3658的观察结果不一致。中子星。我们表明，在对这种弱磁系统进行建模时，必须考虑多极磁矩，辐射阻力和广义相对论通常被忽略的影响。仅使用非常一般的假设，我们就获得了SAX J1808.4-3658偶极子磁矩的μ（1-10）×1026 G cm3的可靠估计，这意味着恒星赤道表面的偶极子场约为108-109 G 。因此，我们推断，在其增生阶段结束后，该源将变为正常的毫秒无线电脉冲星。最后，我们将这种脉冲星的物理性质与低质量X射线双星中的非脉冲弱磁中子星的物理性质进行了比较，并认为后者缺乏相干脉动并不一定意味着这些中子星具有显着不同磁场强度来自SAX J1808.4-3658。
展开▼
