Gravitational lensing by extremal rotating black holes in the strong deflection limit
Abstract
In the strong deflection regime, light rays passing close to an astrophysical black hole may remain trapped near unstable photon orbits for a long time before escaping to infinity. The traditional strong-deflection limit, which accurately describes the logarithmic divergence of the deflection angle for spherically symmetric and slowly rotating black holes, breaks down when the relevant prograde critical photon orbit coincides with the degenerate horizon of an extremal rotating black hole. We present a new strong deflection limit expansion for this horizon critical orbit, covering a general class of extremal rotating black holes. We show that the deflection angle exhibits a stronger power-law divergence in addition to the logarithmic divergence. For an adequate description of higher-order images, additional terms in the expansion must be retained. We first study prograde gravitational lensing in the equatorial plane and then extend the analysis to quasi-equatorial motion, which allows us to calculate the magnification of the higher-order images and the position of the caustic points. We finally apply the general framework to explicit examples, including the Kerr, Kerr-Newman, and Kerr-Sen metrics.
Versions
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BibTeX
@misc{feleppa2026gravitational,
title = {{Gravitational lensing by extremal rotating black holes in the strong deflection limit}},
author = {Fabiano Feleppa and Valerio Bozza and Welmoed Marit de Graaf},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2606.29418},
eprint = {2606.29418},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.29418},
}
RIS
TY - UNPB
TI - Gravitational lensing by extremal rotating black holes in the strong deflection limit
AU - Fabiano Feleppa
AU - Valerio Bozza
AU - Welmoed Marit de Graaf
PY - 2026
DA - 2026/10/12
PB - arXiv
DO - 10.48550/arxiv.2606.29418
UR - https://arxiv.org/abs/2606.29418
AB - In the strong deflection regime, light rays passing close to an astrophysical black hole may remain trapped near unstable photon orbits for a long time before escaping to infinity. The traditional strong-deflection limit, which accurately describes the logarithmic divergence of the deflection angle for spherically symmetric and slowly rotating black holes, breaks down when the relevant prograde critical photon orbit coincides with the degenerate horizon of an extremal rotating black hole. We present a new strong deflection limit expansion for this horizon critical orbit, covering a general class of extremal rotating black holes. We show that the deflection angle exhibits a stronger power-law divergence in addition to the logarithmic divergence. For an adequate description of higher-order images, additional terms in the expansion must be retained. We first study prograde gravitational lensing in the equatorial plane and then extend the analysis to quasi-equatorial motion, which allows us to calculate the magnification of the higher-order images and the position of the caustic points. We finally apply the general framework to explicit examples, including the Kerr, Kerr-Newman, and Kerr-Sen metrics.
ER -