On the perturbation solutions to the Blandford-Znajek mechanism in the Kerr-Sen black hole*

Abstract We investigated the steady, axisymmetric, force-free magnetosphere of the Kerr–Sen black hole (BH) within the framework of the Einstein–Maxwell–dilaton–axion (EMDA) theory. By perturbatively solving the nonlinear Grad–Shafranov (GS) equation, we determined the magnetic field configuration and quantified the influence of the dilaton parameter r 2 on the energy extraction rate and radiative efficiency. Our results show that both the energy extraction power and radiative efficiency increase with r 2 , exceeding the values of the standard Kerr BH; meanwhile, the extraction efficiency remains consistent with the Kerr case. In addition, we performed χ 2 statistical analysis using observational data from six binary BH systems. The results indicate that the Kerr BH currently provides a better fit for bulk Lorentz factors Γ = 2 and 5.