Plasma instability splits axion-photon conversion lines

Abstract: The axion coupling to electromagnetism may leave signatures of uncertain nature in astrophysical spectra; one is thought to be a single line centered at the axion frequency $$ω= \sqrt{\left(m_a c^{2/\hbar\right)²} + c² k_a<sup>2}$$, with $m_a c<sup>2/\hbar$ the axion Compton frequency and $\hbar k_a$ its momentum. If a parametric plasma instability is excited, this signature is transformed into a set of lines, which may be proportionally spaced and linearly polarized. A Crab pulsar signal at a frequency near recent cosmological predictions for the axion Compton frequency has some qualitative similarity to this signature, though this similarity is not evidence that the signal is caused by axions.