Axion Star and NS

Summary Table

Category	Progenitor	Type	Energy Mechanism	Emission Mechanism	Counterparts									References	Brief Comments
					LF Radio	HF Radio	Microwave	Terahertz	Optical/IR	X-rays	Gamma-rays	Gravitational Waves	Neutrinos
Collision / Interaction	Axion Star and NS	Single	Electron oscillation	Coherent dipole radiations	Yes	No	No	No	No	No	No	No	No	arXiv:1410.4323,arXiv:1412.7825,arXiv:1512.06245,arXiv:1810.07270, http://adsabs.harvard.edu/abs/2016PhRvD..94j3004R	None

Definitions: LF Radio (3 MHz to 3 GHz); HF Radio (3 GHz to 30 GHz); Microwave (30 to 300 GHz)

Model Description

In the presence of a magnetic field, axions have been shown to produce radiation by generating an oscillating electric field, causing nearby electrons to radiate coherently. The radiation produced when an axion star collides with a NS has been shown to be consistent with non-repeating FRBs. As the axion star moves through the magnetosphere of the neutron star, a time-dependent electric dipole moment is induced, forcing free electrons above the surface of the NS to oscillate harmonically. This generates coherent dipole radiation with a frequency determined by the axion mass. The theory is shown to be robust to the effects of tidal disruption because the coherence of the axion star is robust; the number density of the axions in the axion star is huge.

Observational Constraints

A defining feature of the model is that the intrinsic FRB emission frequency is finite, and the observed spectral broadening is due to thermal Doppler effects. The emission is also expected to be linearly polarized because it is coherent dipole radiation. A two-component profile may be observed if the axion star collides with a binary NS system. No counterparts are expected. The significant broadening of FRBs, their linear polarization and the large range of frequencies at which FRBs have been detected is consistent with these theories.