Extended axion electrodynamics: Anomalous dynamo-optical response
  induced by gravitational pp-waves
  We extend the Einstein-Maxwell-axion theory including into the Lagrangian
cross-terms of the dynamo-optical type, which are quadratic in the Maxwell
tensor, linear in the covariant derivative of the macroscopic velocity
four-vector, and linear in the pseudoscalar (axion) field or its gradient
four-vector. We classify the new terms with respect to irreducible elements of
the covariant derivative of the macroscopic velocity four-vector of the
electromagnetically active medium: the expansion scalar, acceleration
four-vector, shear and vorticity tensors. Master equations of the extended
axion electrodynamics are used for the description of the response of an
axionically active electrodynamic system, induced by a pp-wave gravitational
background. We show that this response has a critical character, i.e., the
electric and magnetic fields, dynamo-optically coupled to the axions, grow
anomalously under the influence of the external pp-wave gravitational field.
