Superradiant bound states of charged scalars around Melvin-Kerr black holes

Abstract

We investigate superradiant bound states of a charged, massive scalar in the Melvin-Kerr spacetime, a rotating black hole immersed in an external magnetic field, working in the weak-field regime $bM\ll 1$. We adopt a gauge with $A_t\to 0$ at spatial infinity, thereby removing spurious $1/a$ artifacts in the $a\to 0$ limit, and show that the Klein–Gordon equation separates so that the radial dynamics can be cast in Schrödinger-like form. The effective potential exhibits a universal large-$r$ tail with both $1/r$ and $1/r^2$ corrections controlled by the magnetic coupling $bmq$. Using a far-zone Whittaker reduction we derive a simple trapping criterion that captures when a potential well forms outside the horizon. Combining this with the magnetically shifted superradiant band at the horizon and with the decay requirement at infinity yields an instability window in which superradiantly amplified quasi-bound states exist. The criterion reduces continuously to the standard Kerr bound in the limit $b\to 0$, which provides a nontrivial consistency check. Numerical profiles of the effective potential illustrate parameter sets with and without trapping, clarifying how the magnetic field and the azimuthal number compete to enhance or to quench the instability.