Observability of Coulomb-assisted quantum vacuum birefringence

We consider the scattering of an x-ray free-electron laser (XFEL) beam on the superposition of a strong magnetic field {\mathbf{B}}_{\mathrm{ext}}with the Coulomb field {\mathbf{E}}_{\mathrm{ext}}of a nucleus with charge number Z In contrast to Delbr\"uck scattering (Coulomb field only), the magnetic field {\mathbf{B}}_{\mathrm{ext}}introduces an asymmetry (i.e., polarization dependence) and renders the effective interaction volume...