Anomalous character of the axion-photon coupling in a magnetic field
  distorted by a pp-wave gravitational background
  We study the problem of axion-photon coupling in the magnetic field
influenced by gravitational radiation. We focus on exact solutions to the
equations for axion electrodynamics in the pp-wave gravitational background for
two models with initially constant magnetic field. The first model describes
the response of an initially constant magnetic field in a gravitational-wave
vacuum with unit refraction index; the second model is characterized by a
non-unit refraction index prescribed to the presence of ordinary and/or dark
matter. We show that both models demonstrate anomalous behavior of the
electromagnetic field generated by the axion-photon coupling in the presence of
magnetic field, evolving in the gravitational wave background. The role of
axionic dark matter in the formation of the anomalous response of this
electrodynamic system is discussed.
