Black hole correspondences of quasi-topological gravity : Shadows, quasinormal modes, graybody factors
Within the framework of quasi-topological gravity, we systematically investigate the correspondences among three classes of observables---the shadow, the quasinormal modes, and the graybody factors---of regular black holes in $D = 5$ dimensions. Using the WKB approximation and GrayHawk numerical integration, we test the applicability of the GBF--QNM correspondence and the shadow--GBF correspondence in the low-mode ($l=2$) and higher-dimensions ($D=5$): the difference between the GBF--QNM correspondence and the numerical results remains small in magnitude; we apply the known Langer correction (replacing $l$ by the effective angular momentum $\kappa = \sqrt{l(l+D-3)} \simeq l + (D-3)/2$), which reduces the error of the shadow--GBF correspondence at low modes to a level comparable with that of the GBF--QNM correspondence. Our work extends the shadow--GBF correspondence to higher-dimensional regular black holes, providing a viable route to predict the black hole spectrum and the Hawking radiation profile from a single observable, and thereby to realize multi-messenger tests of gravity.