  Axions are pseudo-scalar particles, those arise because of breaking of Peccei
Queen (PQ) symmetry. Axions have a tree level coupling to two photons. As a
consequence there exists a tree level coupling of axion to photon in a magnetic
field. However, in an external magnetic field, there exists a new loop induced,
axion photon vertex, that gives rise to axion photon coupling. The strength of
the tree level axion photon coupling in magnetic field is known to be model
dependent. However in a magnetic field, the new loop induced coupling has some
interesting features. This note discusses the new axion photon vertex in a
magnetized medium and the corrections arising from there. The magnitude of the
correction to axion photon coupling, because of magnetized vacuum and matter is
estimated in this note. While making this estimate we note that the form of the
axion photon vertex is related to the axial polarization tensor. This vertex is
shown to satisfy the Ward identity. The coupling is shown to have a momentum
dependent piece in it. Astrophysical importance of this extra modification is
also pointed out.
