Vacuum polarization effects in the background of a deformed compact object and implications for photon velocity

This paper studies the impact of vacuum polarization on light propagation in the background of a distorted, deformed compact object. Focusing on a spacetime containing two quadrupole parameters associated with the central object and external fields, we explore how these parameters influence observable effects as dynamical degrees of freedom. In this setup, we investigate electromagnetic birefringence, noting distinct polarization-dependent photon velocity variations and gravitational lens effects. Although current resolution may limit detection, future high-precision observations could reveal these quantum electrodynamics (QED) induced birefringence effects, advancing our understanding of vacuum birefringence in astrophysical contexts. We further analyze the dependence of shadow properties on the model's variables, using observational data from Sgr A*.