Explanation of Detailed Spectral Properties of FRBs by Axion Star Model

Abstract

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.