  The changes in the cosmic microwave background (CMB) spectrum seen as an
increase of temperature due to a strong magnetic field are determined and their
influence on the polarization of the radiation is exhibited. The effect is due
to the coupling of the CMB photons to the magnetic field in the QED vacuum via
the interaction with virtual pairs. In spite of the fact that the distortion of
the CMB spectrum for magnetic fields that exist in the vicinity of magnetars is
quite large, this effect is very difficult to detect at present because the
required angular resolutions is not yet available.
