Photons in a cold axion background and strong magnetic fields:
  polarimetric consequences
  In this work we analyze the propagation of photons in an environment where a
strong magnetic field (perpendicular to the photon momenta) coexists with an
oscillating cold axion background with the characteristics expected from dark
matter in the galactic halo. Qualitatively, the main effect of the combined
background is to produce a three-way mixing among the two photon polarizations
and the axion. It is interesting to note that in spite of the extremely weak
interaction of photons with the cold axion background, its effects compete with
those coming from the magnetic field in some regions of the parameter space. We
determine (with one plausible simplification) the proper frequencies and
eigenvectors as well as the corresponding photon ellipticity and induced
rotation of the polarization plane that depend both on the magnetic field and
the local density of axions. We also comment on the possibility that some of
the predicted effects could be measured in optical table-top experiments.
