We investigate the superradiant instability of Kerr-Newman black holes in the presence of a massive, charged scalar field using the Vieira-Bezerra-Kokkotas (VBK) method. We study the solutions of the exact polynomial condition for quasibound state frequencies and determine the domain of superradiant instability in parameter space without relying on the hydrogenic approximation or numerics. We derive the minimum scalar mass needed for quasibound states to exist, and identify the precise overlap region between the quasibound and superradiant conditions where instability can occur. We obtain perturbative and exact analytic expressions for the instability boundaries and growth rates, and clarify their relation to previous numerical results. Our analysis reveals how the instability region shifts from nearly neutral Kerr black holes for light fields to highly charged near-extremal Kerr-Newman black holes for heavier fields, while remaining absent in the Reissner-Nordström limit.

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• Classical black holes
• General relativity
• General relativity equations & solutions