Quasi-normal modes and shadows of scale-dependent regular black holes

Abstract In this paper we investigate how a regular scale-dependent black hole, characterized by a single extra parameter ϵ , behaves under perturbations by a test field (quasi-normal modes) and under light imaging (shadows) in a four-dimensional space-time background. On the quasi-normal modes side, we study how it responds to scalar and Dirac perturbations. To do this, we implement the well known WKB semi-analytic method of 6th order for obtaining the quasi-normal frequencies. We derive analytic expressions for the quasinormal frequencies beyond the eikonal limit for both scalar and Dirac perturbations, finding excellent agreement with the WKB approximation. We discuss the behavior of the real and imaginary parts of the quasi-normal modes for different values of the parameter ϵ and the overtone n and multipole ℓ numbers. On the black hole imaging side, we ray-trace the geometry and illuminate it with a thin-accretion disk. Choosing ϵ = 1.0 we compute the size of the central brightness depression and generate full images of the black hole. We discuss the features (i.e. luminosity) of successive photon rings through the Lyapunov exponent of nearly-bound, unstable geodesics. Furthermore we use the correspondence (in the limit ℓ ≫ n ) between quasi-normal mode frequencies and unstable bound light orbits to infer the numerical values of the latter using the former and find a remarkable accuracy of the correspondence in providing the right numbers. Our results support the usefulness of this correspondence in order to perform cross-tests of black holes using these two messengers.