Axion Star and NS

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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 -- Yes -- -- -- -- -- -- -- --,, 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 radiation with a frequency determined by the axion mass an effect which could be even larger, if one considers the electric dipole moment induced in the neutrons interior to the NS. The theory is shown to be robust to the effects of tidal disruption, however this has been disputed.

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 circularly polarized. 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 at odds with these theories.