In this talk I will discus vacuum birefringence and how a mixture of a strong magnetic field and an additional Coulomb interaction can modify the effect. In particular we consider the scattering of an x-ray free-electron laser (XFEL) beam on the superposition of a strong magnetic field with the Coulomb field of a nucleus.

In contrast to Delbrück scattering (Coulomb field only), the magnetic field introduces an asymmetry (i.e., polarization dependence) and renders the effective interaction volume quite large, while the nuclear Coulomb field facilitates a significant momentum transfer. I will talk about the cross section of this scattering process and some experimental scenarios.  The effect might be observable in the near future at facilities such as the Helmholtz International Beamline for Extreme Fields at the European XFEL.