  We have proposed a generation mechanism of non repeating ( repeating ) fast
radio bursts: They arise by axion star collisions with neutron stars (
accretion disks of galactic black holes ). The axion star as coherent state of
axions with mass $m_a$ generates homogeneous electric field oscillating with
frequency $m_a/2\pi$ under strong magnetic fields. The field makes electrons
coherently oscillate and emit the coherent dipole radiations ( FRBs ). The
radiations stop when thermal fluctuations produced by the oscillation disturb
the coherent oscillations. Thus, the duration of the FRBs is determined by the
time scale of the thermalization; it is much shorter than $1$ms. Line spectra
of the dipole radiations are broadened by the thermal effects. The thermally
broaden spectra have a feature that the bandwidths $\delta\nu$ are proportional
to their center frequencies $\nu_c$. The presence of various center frequencies
( $1.2$GHz$\sim $$7$GHz ) in repeating FRB 121102 is attributed to Doppler
shifts owing to the various relativistic velocities of the accretion disk. On
the other hand, non repeating FRBs do not show such variety of the center
frequencies. They come from the surfaces of neutron stars. The Doppler shift
also makes the durations of the bursts with higher frequencies become shorter.
Because the magnetic fields of the neutron stars are supposed to be stronger
than those of the accretion disks, the peak flux densities of non repeating
FRBs are larger than those of repeating FRB 121102. The strong magnetic fields
of the neutron stars also lead to much wide bandwidths of non repeating FRBs,
which are over the 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.
