# Near-horizon quasinormal modes of charged scalar around a general spherically symmetric black hole

## TL;DR

This paper investigates the near-horizon quasinormal modes of charged scalar fields around static spherically symmetric black holes, combining numerical and analytical methods to reveal universal patterns and dependencies on black hole parameters.

## Contribution

It provides an analytical formula for near-horizon QNMs in general static spherically symmetric black holes and explores their behavior through numerical analysis in dRGT massive gravity.

## Key findings

- QNMs form a tower pattern with real parts depending on charge product
- Imaginary parts depend solely on surface gravity
- Four QNM towers converge to asymptotic values with equal spacing

## Abstract

We study the quasinormal modes (QNMs) of charged scalar in the static spherically symmetric black hole background near the event and cosmological horizon. Starting with numerical analysis of the QNMs of black hole in the dRGT massive gravity, the mathematical tool called the Asymptotic Iteration Method~(AIM) is used to calculate the quasinormal frequencies. The parameters such as the mass and charge of the black hole, the cosmological constant, the coefficient of the linear term from massive gravity $\gamma$, and the mass of the scalar are varied to study the behavior of the QNMs. We found the tower pattern of the near-horizon quasinormal frequencies from the numerical results by AIM where the real parts depend only on product of the charge of the black hole and the scalar field and the imaginary parts depend only on the surface gravity. To confirm the numerical finding, we analytically determine the exact QNMs of the charged scalar near the horizons of any static spherically symmetric black hole background in the simple universal forms; $\omega = \displaystyle{\frac{qQ}{r_h}} + i \kappa_h n$ and $\omega = \displaystyle{\frac{qQ}{r_c}} + i |\kappa_c| n$ where $n$ is a non-positive integer and $\kappa_{h,c}$ is the surface gravity, for the event and cosmological horizon respectively. Extending our analysis, we also compute the four towers of the near-horizon QNMs that can reach the far region. The four kinds of QNMs converge to certain asymptotic values with equally spacing imaginary parts and the real parts proportional to $qQ/r_{h,c}$. These modes do not match with the all-region~(WKB) modes of the real background since they are originated from the linearly approximated metric.