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 $\omega =\sqrt{{\left(m_a{c}^2/\hslash \right)}^2+c^2{k}_a^2}$, with $m_a{c}^2/\hslash$ the axion Compton frequency and $\hslash k_a$ its momentum. If a parametric plasma instability is excited, this signature is transformed into a 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.