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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.

Explanation of detailed spectral properties of fast radio bursts by the axion star model