Kerr black hole in presence of force-free magnetic field

Abstract

We extend the study of force-free magnetospheres from non-rotating to rotating black holes \cite{Sheikhahmadi} and investigate the influence of a force-free magnetic field on the geometry around a Kerr black hole. Using the Newman-Penrose formalism, we explicitly construct the electromagnetic field strength tensor in the Kerr background and compute the corresponding stress-energy tensor. The resulting metric perturbation is then obtained by solving the linearised Einstein equations. In this modified geometry, we analyse key observables of thin accretion disks, including the innermost stable circular orbit (ISCO), effective potential, energy flux, temperature, and efficiency parameter. Our results demonstrate that magnetic backreaction significantly alters the spacetime near the black hole, with important consequences for accretion physics and jet-launching mechanisms such as the Blandford-Znajek process. This work underlines the essential role of magnetic fields in shaping relativistic astrophysical environments.