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We have proposed a generation mechanism of non repeating ( repeating ) fastradio bursts: They arise by axion star collisions with neutron stars (accretion disks of galactic black holes ). The axion star as coherent state ofaxions with mass $m_a$ generates homogeneous electric field oscillating withfrequency $m_a/2\pi$ under strong magnetic fields. The field makes electronscoherently oscillate and emit the coherent dipole radiations ( FRBs ). Theradiations stop when thermal fluctuations produced by the oscillation disturbthe coherent oscillations. Thus, the duration of the FRBs is determined by thetime scale of the thermalization; it is much shorter than $1$ms. Line spectraof the dipole radiations are broadened by the thermal effects. The thermallybroaden spectra have a feature that the bandwidths $\delta\nu$ are proportionalto their center frequencies $\nu_c$. The presence of various center frequencies( $1.2$GHz$\sim $$7$GHz ) in repeating FRB 121102 is attributed to Dopplershifts owing to the various relativistic velocities of the accretion disk. Onthe other hand, non repeating FRBs do not show such variety of the centerfrequencies. They come from the surfaces of neutron stars. The Doppler shiftalso makes the durations of the bursts with higher frequencies become shorter.Because the magnetic fields of the neutron stars are supposed to be strongerthan those of the accretion disks, the peak flux densities of non repeatingFRBs are larger than those of repeating FRB 121102. The strong magnetic fieldsof the neutron stars also lead to much wide bandwidths of non repeating FRBs,which are over the extent of the receiver frequency range. The spectralfeatures of the recently discovered new repeating FRB 180814.J0422+75 arecoincident with our general analyses of the repeating FRB 121102.

Explanation of Detailed Spectral Properties of FRBs by Axion Star Model