Horizon-Evanescent Scalar Clouds from Coupled Rotation and Magnetic Fields around Black Holes
Abstract
We show that black-hole rotation and an external magnetic field can jointly generate a distinct class of scalar clouds. Using the Kerr-Bertotti-Robinson geometry as a separable laboratory for magnetized rotating black holes, we study a charged massive scalar field and map the radial Klein-Gordon equation into a one-dimensional Schrödinger-like form. The magnetic coupling shifts the near-horizon dispersion relation and realizes a positive horizon gap: a sufficient near-horizon criterion under which the horizon wavenumber becomes purely imaginary in a finite frequency band below the usual kinematic synchronization frequency. In this band the physical horizon boundary condition is no longer a propagating ingoing wave, but a regular exponentially decaying state. This rotation-magnetic-field mechanism quenches the superradiant flux and supports horizon-decaying scalar clouds (Type-II), distinct from the usual synchronized propagating clouds (Type-I). Matched asymptotic expansions and numerical shooting solutions are used to exhibit both branches and their spatial profiles. Thus the Kerr-Bertotti-Robinson solution is not an isolated curiosity, but an explicit realization of a broader positive-gap criterion for horizon-evanescent stationary configurations that are absent in isolated Kerr systems.
Versions
Related
- Coupled membrane response and horizon dissipation in four-dimensional Einstein-scalar-Gauss-Bonnet gravity
- Discrete quality-factor control in a side-coupled photonic crystal microcavity: evanescent Bloch tunnelling and the finite-cell correction
- Quasinormal modes of massive scalar perturbations in slow-rotation bumblebee black holes with traceless conformal electrodynamics
- Second-order slow rotation of wormholes in Einstein-scalar-Gauss-Bonnet gravity
- Near-Horizon Scalar-Wave Sampling: Rindler Bounds and Schwarzschild Comparison
BibTeX
@misc{xu2026horizonevanescent,
title = {{Horizon-Evanescent Scalar Clouds from Coupled Rotation and Magnetic Fields around Black Holes}},
author = {Hengyu Xu and Haowei Chen and Shao-Jun Zhang},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2606.27958},
eprint = {2606.27958},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.27958},
}
RIS
TY - UNPB
TI - Horizon-Evanescent Scalar Clouds from Coupled Rotation and Magnetic Fields around Black Holes
AU - Hengyu Xu
AU - Haowei Chen
AU - Shao-Jun Zhang
PY - 2026
DA - 2026/09/29
PB - arXiv
DO - 10.48550/arxiv.2606.27958
UR - https://arxiv.org/abs/2606.27958
AB - We show that black-hole rotation and an external magnetic field can jointly generate a distinct class of scalar clouds. Using the Kerr-Bertotti-Robinson geometry as a separable laboratory for magnetized rotating black holes, we study a charged massive scalar field and map the radial Klein-Gordon equation into a one-dimensional Schrödinger-like form. The magnetic coupling shifts the near-horizon dispersion relation and realizes a positive horizon gap: a sufficient near-horizon criterion under which the horizon wavenumber becomes purely imaginary in a finite frequency band below the usual kinematic synchronization frequency. In this band the physical horizon boundary condition is no longer a propagating ingoing wave, but a regular exponentially decaying state. This rotation-magnetic-field mechanism quenches the superradiant flux and supports horizon-decaying scalar clouds (Type-II), distinct from the usual synchronized propagating clouds (Type-I). Matched asymptotic expansions and numerical shooting solutions are used to exhibit both branches and their spatial profiles. Thus the Kerr-Bertotti-Robinson solution is not an isolated curiosity, but an explicit realization of a broader positive-gap criterion for horizon-evanescent stationary configurations that are absent in isolated Kerr systems.
ER -