  We have proposed a model of non repeating fast radio bursts ( FRBs ); the
collisions between axion stars and neutron stars generate the bursts. In this
paper, we propose a model of repeating FRBs which shows that they arise from
the several collisions between magnetized accretion disk of a black hole and an
axion star orbiting the black hole. There would be many axions stars condensing
as dark matter in an early stage galaxy so that such collisions arises
repeatedly. The radiations are emitted by coherent oscillations of electrons in
the accretion disk. The oscillations are caused by oscillating electric fields,
which are induced by axion stars under strong magnetic field $\sim
O(10^{11})$G. The emissions are terminated by the thermal fluctuations of the
electrons which result from the thermalization of the oscillation energies.
Although the radiations are monochromatic, the thermal Doppler effects broaden
the spectral lines; their spectra $S(\nu)$ are given by $S(\nu) \propto
\exp(-(\nu-\nu_c)^2/2(\delta\nu)^2)$ with the center frequency $\nu_c$. The
bandwidth $\delta\nu=\nu_c\sqrt{T_c/m_e}$ with electron mass $m_e$ is
determined by the critical temperature $T_c$ at which the thermal fluctuations
terminate the coherent emissions. The observed bandwidths
$(3\sim5)\times10^2$MHz are originated from the strong magnetic fields $\sim
O(10^{11})$G. With such strong magnetic fields, large amount of the burst
energies are produced. Various center frequencies $\nu_c$ of the bursts in the
repeating FRB 121102 come from the various rotation speeds in the disk which
make an intrinsic frequency of the bursts being Doppler shifted. The recent
observation showing an extreme magneto-ionic environment around the source of
FRB 121102 supports our model.
