Gravitational lensing by charged black hole with global monopole in the strong field limit

Abstract: We investigate gravitational lensing near a charged black hole with a global monopole in the strong field regime, focusing on the combined effects of the global monopole and black hole charge on key observables in gravitational lensing both analytically and numerically. Our results reveal that the dependence of the angular separation on charge is intricately tied to the deficit angle caused by the global monopole. In particular, we identify three critical values of the global monopole parameter that determine whether the angular separation increases monotonically, decreases monotonically, or exhibits extrema as the charge varies. A similar complex dependence is found for the flux ratio as a function of the deficit angle, and for the magnification of the first relativistic image as a function of charge. These behaviors contrast sharply with the monotonic changes observed in the absence of either a global monopole or charge. Our findings highlight that the effects of the charge and global monopole on gravitational lensing cannot be described as simple additive contributions. Instead, their combined effects lead to a rich and interdependent behavior that enhances our understanding of strong-field gravitational lensing. While the charge and global monopole are expected to be small in typical astrophysical contexts, the results presented here could be experimentally explored in analog gravity systems, where these parameters are not constrained. This opens the door to potential experimental verification of the phenomena predicted in this study.