Vacuum birefringence at the Gamma Factory

We explore the perspectives of studying vacuum birefringence at the Gamma Factory. To this end, we assess in detail the parameter regime which can be reliably analyzed resorting to the leading contribution to the Heisenberg-Euler effective Lagrangian. We explicitly show that -- contrarily to naive expectations -- this approach allows for the accurate theoretical study of quantum vacuum signatures up to fairly large photon energies. The big advantage of this parameter regime is the possibility of studying the phenomenon in experimentally realistic, manifestly inhomogeneous pump and probe field configurations. Thereafter, we focus on two specific scenarios giving rise to a vacuum birefringence effect for traversing gamma probe photons. In the first scenario the birefringence phenomenon is induced by a quasi-constant static magnetic field. In the second case it is driven by a counter-propagating high-intensity laser field.