论文标题
年轻无线电脉冲星的制动索引：理论观点
Braking indices of young radio pulsars: theoretical perspective
论文作者
论文摘要
最近，Parthsarathy等人。分析了85个年轻无线电脉冲星的长期定时观察。他们发现11个对象具有$ \ sim 10-100 $的制动索引，远离经典值$ n = 3 $。他们还指出，$ n $的测量值与无线电脉冲星的特征年龄之间存在温和的相关性。在本文中，我们系统地分析了大型制动指数的可能物理起源。我们发现，这些测量值的一小部分可能是由于脉冲星的超宽伴侣或动力的重力加速所致。剩余的制动指数不能用脉冲星斜角演化，也不能通过poloidal磁场的复杂高阶多极结构来解释。最合理的解释是对poloidal偶极磁场的衰减，该磁场在某些年轻物体中以时间尺度运行$ \ sim 10^4-10^5 $年，但在其他无线电脉冲星中的时间尺度明显更长。这种衰减可以解释$ n $的幅度，以及$ n $和特征年龄之间的一些相关性。衰减可能是由于某些孤立无线电脉冲星的外壳中增强的晶体杂质引起的，或者更有可能是由于低质量中子星的冷却缓慢而与电子散射相关的增强电阻率。如果这种效果确实是快速磁场衰减表现为大型制动指数的主要原因，那么我们预测，与$ n \ 3 $的脉冲星相比，具有较大制动指数的脉冲星比较热。
Recently, Parthsarathy et al. analysed long-term timing observations of 85 young radio pulsars. They found that 11 objects have braking indices ranging $\sim 10-100$, far from the classical value $n=3$. They also noted a mild correlation between measured value of $n$ and characteristic age of a radio pulsar. In this article we systematically analyse possible physical origin of large braking indices. We find that a small fraction of these measurements could be caused by gravitational acceleration from an unseen ultra-wide companion of a pulsar or by precession. Remaining braking indices cannot be explained neither by pulsar obliquity angle evolution, nor by complex high-order multipole structure of the poloidal magnetic field. The most plausible explanation is a decay of the poloidal dipole magnetic field which operates on a time scale $\sim 10^4-10^5$ years in some young objects, but has significantly longer time scale in other radio pulsars. This decay can explain both amplitude of measured $n$ and some correlation between $n$ and characteristic age. The decay can be caused by either enhanced crystal impurities in the crust of some isolated radio pulsars, or more likely, by enhanced resistivity related to electron scattering off phonons due to slow cooling of low-mass neutron stars. If this effect is indeed the main cause of the rapid magnetic field decay manifesting as large braking indices, we predict that pulsars with large braking indices are hotter in comparison to those with $n\approx 3$.
