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Black hole photon ring beyond General Relativity: an integrable parametrization
Jibril Ben Achour, Eric Gourgoulhon, Hugo Roussille
https://arxiv.org/abs/2506.09882

Black hole photon ring beyond General Relativity: an integrable parametrization
In recent years, the shape of the photon ring in black holes images has been argued to provide a sharp test of the Kerr hypothesis for future black hole imaging missions. In this work, we confront this proposal to beyond Kerr geometries and investigate the degeneracy in the estimations of the black hole parameters using the circlipse shape proposed by Gralla and Lupsasca. To that end, we consider a model-independent parametrization of the deviations to the Kerr black hole geometry, dubbed Kerr …