Greybody Factor, Resonant Frequencies, and Entropy Quantization of Charged Scalar Fields in the Kerr-EMDA Black Hole

TL;DR

This paper analytically investigates charged scalar perturbations in Kerr-EMDA black holes, revealing new physics due to electromagnetic coupling, deriving resonant frequencies, entropy quantization, and computing the greybody factor with superradiance effects.

Contribution

It provides the first analytical solutions for charged scalar fields in Kerr-EMDA black holes, highlighting the impact of electromagnetic coupling on spectra and thermodynamics.

Findings

Resonant frequencies have universal imaginary part spacing of 1/(2M).

Entropy quantum diverges at extremality, contrasting with other black hole models.

First analytical greybody factor computed, showing superradiant amplification.

Abstract

We study charged massive scalar field perturbations on the rotating black hole (BH) background of Einstein-Maxwell-Dilaton-Axion (EMDA) theory, known as the Kerr-EMDA BH. Starting from the gauge-covariant Klein-Gordon equation (KGE), we perform a full separation of variables and obtain exact analytical solutions for both the angular and radial parts in terms of confluent Heun functions (CHFs). Unlike the earlier neutral scalar treatment by Senjaya and Ponglertsakul [Eur. Phys. J. C \textbf{85}, 352 (2025)], the electromagnetic coupling $q$ fundamentally alters the structure of the Heun parameters and produces qualitatively new physics. Applying the CHF polynomial condition, we derive the resonant frequency spectrum whose imaginary parts are equispaced with $|\Delta\omega_I| = 1/(2M)$, a universal spacing determined solely by the BH mass. Via the Maggiore prescription and the first law of BH thermodynamics, this yields a parameter-dependent entropy quantum $\delta S_{\text{BH}} = 4\pi r_+/(r_+ - r_-)$, which reduces to $4\pi$ for Schwarzschild but diverges at extremality -- {\color{black}in contrast to the universal $2\pi$ obtained for the rotating linear dilaton BH (RLDBH).} We construct the effective potential governing scalar wave scattering and analyze its dependence on the dilaton parameter $D$, rotation $a$, and scalar charge $q$. In the massless uncharged limit, the CHF reduces to the Gauss hypergeometric function, {\color{black}enabling us to compute the first analytical greybody factor (GF) for the Kerr-EMDA geometry; we show that this reduction extends to massless charged scalars, yielding a closed-form GF that captures superradiant amplification.} We examine how the dilaton deformation distinguishes the Kerr-EMDA spectrum from the standard Kerr and Kerr-Newman cases.