[论文解读] A propeller model for the sub-luminous disk state of the transitional millisecond pulsar PSR J1023+0038

Alessandro Papitto, D. F. Torres|arXiv (Cornell University)|Apr 20, 2015

Astrophysics and Cosmic Phenomena参考文献 7被引用 4

一句话总结

该论文提出，过渡毫秒脉冲星PSR J1023+0038的亚发光盘态是由一种螺旋桨机制驱动的，其中快速旋转的中子星磁层将大部分吸积物质排出，在吸积盘-磁层边界形成湍流层，使电子加速至GeV量级能量。该区域的同步辐射与自同步辐射康普顿辐射，结合硬X射线盘辐射，成功再现了观测到的X射线与伽马射线谱，吸积率为1–3×10⁻¹¹ M⊙ yr⁻¹，内盘半径为30–45 km。

ABSTRACT

The discovery of millisecond pulsars switching between states powered either by the rotation of their magnetic field or by the accretion of matter, has recently proved the tight link shared by millisecond radio pulsars and neutron stars in low-mass X-ray binaries. Transitional millisecond pulsars also show an enigmatic intermediate state in which the neutron star is surrounded by an accretion disk, it emits coherent X-ray pulsations, but is sub-luminous in X-rays with respect to accreting neutron stars, and is brighter in gamma-rays than millisecond pulsars in the rotation-powered state. Here, we model the X-ray and gamma-ray emission observed from PSR J1023+0038 in such a state based on the assumption that most of the disk in-flow is propelled away by the rapidly rotating neutron star magnetosphere, and that electrons can be accelerated to energies of a few GeV at the turbulent disk-magnetosphere boundary. We show that the synchrotron and self-synchrotron Compton emission coming from such a region, together with the hard disk emission typical of low states of accreting compact objects, is able to explain the radiation observed in the X-ray and gamma-ray band. The average emission observed from PSR J1023+0038 is modelled by a disk in-flow with a rate of $(1-3) imes10^{-11} M_{\odot}/yr$, truncated at a radius ranging between 30 and 45 km, compatible with the hypothesis of a propelling magnetosphere. We compare the results we obtained with models that rather assume that a rotation-powered pulsar is turned on, showing how the spin down power released in similar scenarios is hardly able to account for the magnitude of the observed emission.

研究动机与目标

• 解释PSR J1023+0038在中间态、盘嵌入态下观测到的亚发光X射线与增强的伽马射线辐射。
• 解决观测到的高伽马射线光度（~10³⁴ erg s⁻¹）与自转能耗功率较低的自转驱动脉冲星之间的矛盾。
• 检验螺旋桨机制（即快速旋转的中子星将吸积物质排出）是否能解释观测到的辐射特征。
• 将螺旋桨模型与状态转换期间瞬态射电脉冲星活动的其他解释区分开来。

提出的方法

• 模拟湍流吸积盘-磁层边界层中的辐射，其中电子在强磁场与离子不稳定性作用下被加速至GeV能量。
• 计算边界层中相对论性电子产生的同步辐射与自同步辐射康普顿（SSC）辐射，假设电子能量分布为幂律谱。
• 引入典型低光度吸积态的硬X射线成分，采用光子指数Γ ≈ 1.5、在100 keV以下无截断的幂律谱。
• 使用吸积盘流入速率为1–3×10⁻¹¹ M⊙ yr⁻¹、在距离中子星30–45 km处截断的模型拟合观测到的谱能分布（SED）。
• 将螺旋桨模型的能量预算与自转驱动脉冲星情景进行比较，表明后者无法解释观测到的伽马射线光度。
• 利用粘性 timescale 论证，将10–50 s量级的X射线变异性解释为质量流入速率与内盘半径的波动。

实验结果

研究问题

• RQ1 螺旋桨机制能否解释PSR J1023+0038中间态下的亚发光X射线辐射与增强的伽马射线辐射？
• RQ2 在螺旋桨模型下，重现观测SED所需的吸积率与内盘半径是多少？
• RQ3 观测到的约10³⁴ erg s⁻¹的伽马射线光度是否与自转驱动脉冲星的自转能耗功率相容？
• RQ4 10–50 s时间尺度上的X射线流量变异性如何与螺旋桨状态下的吸积动力学相关联？
• RQ5 射电辐射与伽马射线辐射能否由单一辐射区域或机制一致解释？

主要发现

• PSR J1023+0038观测到的X射线与伽马射线SED可由湍流吸积盘-磁层边界层中产生的同步辐射与自同步辐射康普顿辐射组合良好再现。
• 所需吸积率为1–3×10⁻¹¹ M⊙ yr⁻¹，盘在30–45 km处截断，与磁层螺旋桨状态一致。
• 约10³⁴ erg s⁻¹的伽马射线光度无法由自转驱动脉冲星解释，因其需将90%的自转能耗功率转化为伽马射线。
• 10–50 s时间尺度的X射线变异性可自然地由质量流入速率与内盘半径的粘性timescale波动解释。
• 射电辐射可能由更宽、光学薄的喷流产生，而高能辐射则源自磁层边界附近紧凑、自吸收的区域。
• 螺旋桨模型为吸积盘、X射线脉动、低X射线光度与高伽马射线输出的共存提供了自洽的解释。