Resumming Kerr Quasinormal-Mode Frequencies: Accuracy and Breakdown Near Extremality
Abstract
Kerr black-hole quasinormal modes are usually computed with numerical methods, but analytic approximations remain useful for identifying the physics that controls different parts of the spectrum. In this paper, we ask whether the divergent, high-order Wentzel-Kramers-Brillouin (WKB) expansion about the peak of the Chandrasekhar-Detweiler potential can be made predictive through Padé and Borel-Padé resummation. We develop two complementary implementations: a semi-analytic slow-rotation expansion in the dimensionless spin a (carried out through 21st WKB order), and a fixed-spin Padé-WKB implementation for the resummed frequency equation (carried out through 41st WKB order). In the slow-rotation regime, the 21st-order resummed expansion is significantly more accurate than the fourth-order approximation found previously. For damped modes at larger spins, the fixed-point iteration agrees well with Leaver's method, reaching fractional errors below 10−7 in the real part of the fundamental m=0 mode at a=0.99. The same strategy fails for modes that approach the zero-damped branch near extremality. We trace this breakdown to the near-horizon structure of the Chandrasekhar-Detweiler potential. As the extremal limit is approached, nearby poles produce rapid variation on the throat scale, so a local Taylor expansion about the potential peak no longer uniformly captures the relevant region.
Versions
Related
- Quasinormal modes of Hayward black holes from Schwarzschild to extremality: a comparative spectral analysis
- Quasinormal Modes Criticality across Black Hole Extremality
- Pole Skipping, Avoided Crossing, and Resonant Excitation in Kerr Quasinormal Modes near Algebraically Special Frequencies
- Quasinormal frequencies and greybody factors for axial perturbations of dilaton-Euler-Heisenberg de Sitter black holes
- Probing black holes extremality bounds with non-extensive entropies
BibTeX
@misc{hu2026resumming,
title = {{Resumming Kerr Quasinormal-Mode Frequencies: Accuracy and Breakdown Near Extremality}},
author = {Jierui Hu and Kent Yagi and Nicolas Yunes},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2607.27043},
eprint = {2607.27043},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2607.27043},
}
RIS
TY - UNPB
TI - Resumming Kerr Quasinormal-Mode Frequencies: Accuracy and Breakdown Near Extremality
AU - Jierui Hu
AU - Kent Yagi
AU - Nicolas Yunes
PY - 2026
DA - 2026/09/23
PB - arXiv
DO - 10.48550/arxiv.2607.27043
UR - https://arxiv.org/abs/2607.27043
AB - Kerr black-hole quasinormal modes are usually computed with numerical methods, but analytic approximations remain useful for identifying the physics that controls different parts of the spectrum. In this paper, we ask whether the divergent, high-order Wentzel-Kramers-Brillouin (WKB) expansion about the peak of the Chandrasekhar-Detweiler potential can be made predictive through Padé and Borel-Padé resummation. We develop two complementary implementations: a semi-analytic slow-rotation expansion in the dimensionless spin a (carried out through 21st WKB order), and a fixed-spin Padé-WKB implementation for the resummed frequency equation (carried out through 41st WKB order). In the slow-rotation regime, the 21st-order resummed expansion is significantly more accurate than the fourth-order approximation found previously. For damped modes at larger spins, the fixed-point iteration agrees well with Leaver's method, reaching fractional errors below 10^−7 in the real part of the fundamental m=0 mode at a=0.99. The same strategy fails for modes that approach the zero-damped branch near extremality. We trace this breakdown to the near-horizon structure of the Chandrasekhar-Detweiler potential. As the extremal limit is approached, nearby poles produce rapid variation on the throat scale, so a local Taylor expansion about the potential peak no longer uniformly captures the relevant region.
ER -