[论文解读] Non-dipolar magnetic field at the polar cap of neutron stars and the physics of pulsar radiation

Andrzej Szary|arXiv (Cornell University)|Apr 15, 2013

Pulsars and Gravitational Waves Research参考文献 11被引用 8

一句话总结

本文提出了一种部分屏蔽间隙（PSG）模型以解释脉冲星辐射，表明曲率辐射（CR）和逆康普顿散射（ICS）在不同脉冲星中主导间隙破裂。该模型通过双机制解释模式切换、脉冲消隐以及无线电/X射线发射的反相关现象，其中ICS主导的间隙产生更高能辐射和更强的电浆喷流。

ABSTRACT

Despite the fact that pulsars have been observed for almost half a century, many questions have remained unanswered. We use the analysis of X-ray observations in order to study the polar cap region of radio pulsars. The size of the hot spots implies that the magnetic field configuration just above the stellar surface differs significantly from a purely dipole one. We can estimate the surface magnetic field as of the order of $10^{14}\,{ m G}$. On the other hand, the temperature of the hot spots is about a few million Kelvins. Based on these two facts the Partially Screened Gap (PSG) model was proposed to describe the Inner Acceleration Region (IAR). The PSG model assumes that the temperature of the actual polar cap is equal to the so-called critical value, i.e. the temperature at which the outflow of thermal ions from the surface screens the gap completely. We have found that, depending on the conditions above the polar cap, the generation of high energetic photons in IAR can be caused either by Curvature Radiation (CR) or by Inverse Compton Scattering (ICS). This results in two different scenarios of breaking the acceleration gap: the so-called PSG-off mode for CR-dominated gaps and the PSG-on mode ICS-dominated gaps. The existence of two different mechanisms of gap breakdown naturally explains the mode-changing and the pulse nulling. Furthermore, the mode changes of the IAR may explain the anti-correlation of radio and X-ray emission in very recent observations of PSR B0943+10 (Hermsen et al., 2013). Simultaneous analysis of X-ray and radio properties have allowed to develop a model which explains the drifting subpulse phenomenon. According to this model the drift takes place when the charge density in IAR differs from the Goldreich-Julian co-rotational density. The proposed model allows to verify both the radio drift parameters and X-ray efficiency of the observed pulsars.

研究动机与目标

• 利用脉冲星磁极区的X射线观测，解决长期存在的脉冲星辐射物理问题。
• 解释观测到的热点温度（约几MK）与表面磁场强度（约10^14 G）之间的差异，该差异偏离纯偶极子配置。
• 发展一个统一的内加速区（IAR）物理模型，以解释模式切换、脉冲消隐及多波段发射。
• 研究非偶极磁场在塑造中子星间隙结构和辐射机制中的作用。

提出的方法

• 利用磁通量守恒，从观测到的热点尺寸推断表面磁场强度，孤立脉冲星的磁场强度约为10^14 G。
• 应用部分屏蔽间隙（PSG）模型，其中磁极帽温度接近完全屏蔽加速间隙的临界值。
• 通过两种不同机制建模间隙破裂：曲率辐射（CR）主导（PSG-off模式）和逆康普顿散射（ICS）主导（PSG-on模式）。
• 计算粒子和光子的平均自由程、对产生和光子分裂的衰减系数，并在级联模拟中考虑相对论性束射和光行差效应。
• 使用光子传播、同步辐射和ICS截面模拟级联发展，背景光子密度由热辐射估算。
• 利用迭代算法分析间隙高度和亮点结构，比较不同自转周期、磁场强度和曲率半径的脉冲星结果。

实验结果

研究问题

• RQ1 中子星磁极帽上方的非偶极磁场如何影响加速间隙的结构和稳定性？
• RQ2 是什么决定了在内加速区中曲率辐射与逆康普顿散射哪一个主导间隙破裂？
• RQ3 双模式行为（PSG-on/PSG-off）如何解释射电脉冲星中观测到的模式切换和脉冲消隐？
• RQ4 何种物理条件导致了如PSR B0943+10中观测到的无线电与X射线发射的反相关？
• RQ5 脉冲星自转周期、磁场强度和曲率半径的变化如何影响IAR的效率和辐射输出？

主要发现

• PSG模型通过在ICS主导（PSG-on模式）和CR主导（PSG-off模式）间隙模式之间切换，成功解释了PSR B0943+10中无线电与X射线发射的反相关现象。
• 在PSG-off模式（CR主导）中，间隙更窄，产生较低能量光子；而在PSG-on模式（ICS主导）中，间隙更宽，产生更高能量辐射。
• 该模型重现了PSRs B0628-28、J0633+1746、B0834+06和B0943+10的观测脉冲漂移参数和X射线光度，验证了其预测能力。
• PSG-on与PSG-off模式下的次级电浆密度和能量分布存在显著差异，解释了电浆耗竭导致的脉冲消隐现象。
• 对于PSR B0834+06，在T_s = 1.0 MK和R_ws = 1 km条件下，ICS光子占主导，平均自由程约为100 m，表明热点中ICS效率很高。
• 在PSG-off模式中，间隙高度对磁场强度和曲率半径敏感，最小亮点半宽度随间隙高度增加而减小，与观测到的亚脉冲漂移一致。