Two-photon emission process in arbitrarily strong magnetic fields

We perform the second-order calculation for an electron in a strong magnetic field which emits two photons during its transition between any Landau level and any lower Landau level. The calculation is valid for any value of magnetic field, and is of particular interest for processes in magnetars, which are pulsars in which the magnetic field is near, or exceeds, the critical value B-cr = 4.414x10(13) G. The dependence of the transition rate on the spin polarization of the electron along the external magnetic field and the linear or circular polarization of the emitted radiation is given explicitly. [S0556-2821(99)00419-1].