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. 
Alexander B. Balakin and Timur Yu. Alpin
gr-qc astro-ph.CO hep-th