Explanation of detailed spectral properties of fast radio bursts by the axion star model
Explanation of detailed spectral properties of fast radio bursts by the axion star model
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DOI：
10.1093/ptep/ptz142
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发表时间：
2020
影响因子：
3.5
通讯作者：
A. Iwazaki
中科院分区：
文献类型：
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作者：
A. Iwazaki
我们提出了一种非重复（重复）快速射电暴（FRBs）的产生机制：它们是由轴子星与中子星（星系黑洞的吸积盘）碰撞产生的。轴子星作为质量为$m_a$的轴子的相干态，在强磁场作用下产生频率为$m_a/2\pi$的均匀电场。该场使电子相干振荡并发射相干偶极子辐射（FRBs）。当振荡被振荡能量的热化扰动时，辐射停止。因此，快速射电暴的持续时间取决于热化的时间尺度。我们发现它可以短于$1$ ms.热效应使偶极子辐射的谱线展宽。热展宽光谱具有带宽$\delta\nu$与中心频率$\nu_{\rm c}$: $\delta\nu \propto \nu_{\rm c}$成正比的特点。因为吸积盘可以以相对论速度运行，所以辐射是多普勒偏移的。这导致在重复FRB 121102中出现各种中心频率（$\nu_{\rm c}=600 \, \text{MHz} \sim 7$ GHz）。另一方面，非重复的快速射电暴在中心频率上没有表现出这种变化。它们来自运动是非相对论性的中子星的表面。多普勒频移也使高频脉冲的持续时间变短。由于中子星的磁场比吸积盘的磁场强，非重复FRB的峰值通量密度大于重复FRB 121102的峰值通量密度。它们还导致非重复快速射电暴的宽频带，覆盖了接收机频率范围的全部范围。最近发现的新重复FRB 180814的光谱特征。J0422+75与我们对重复FRB 121102的总体分析是一致的。
We have proposed a generation mechanism for non-repeating (repeating) fast radio bursts (FRBs): They arise by axion star collisions with neutron stars (accretion disks of galactic black holes). The axion star, as a coherent state of axions with mass $m_a$, generates a homogeneous electric field oscillating with frequency $m_a/2\pi$ under strong magnetic fields. The field makes electrons oscillate coherently and emit coherent dipole radiation (FRBs). The radiation stops when the oscillations are disturbed by the thermalization of the oscillating energies. Thus, the duration of the FRB is determined by the time scale of the thermalization. We show that it can be shorter than $1$ ms. The line spectra of the dipole radiation are broadened by the thermal effects. The thermally broadened spectra have the feature that the bandwidths $\delta\nu$ are proportional to their center frequencies $\nu_{\rm c}$: $\delta\nu \propto \nu_{\rm c}$. Because the accretion disks can orbit with relativistic velocities, the radiation is Doppler shifted. This leads to the presence of various center frequencies ($\nu_{\rm c}=600 \, \text{MHz} \sim 7$ GHz) in repeating FRB 121102. On the other hand, non-repeating FRBs do not show such a variety in the center frequencies. They come from the surfaces of neutron stars whose motions are non-relativistic. The Doppler shift also makes the duration of bursts with higher frequencies become shorter. Because the magnetic fields of the neutron stars are stronger than those of the accretion disks, the peak flux densities of non-repeating FRBs are larger than those of repeating FRB 121102. They also lead to wide bandwidths of non-repeating FRBs, which cover the full extent of the receiver frequency range. The spectral features of the recently discovered new repeating FRB 180814.J0422+75 are coincident with our general analyses of the repeating FRB 121102.