Shadow Geometry of Kerr Naked Singularities

Abstract

Direct imaging of supermassive black holes (SMBHs) at event horizon scale resolutions, as recently done by the Event Horizon Telescope, allows for testing alternative models to SMBHs such as Kerr naked singularities (KNSs). We demonstrate that the KNS shadow can be closed, open, or vanishing, depending on the spins and observational inclination angles. We study the critical parameters where the KNS shadow opens a gap, a distinctive phenomenon that does not happen with the black hole shadow. We show that the KNS shadow can only be closed for dimensionless spin $a\lesssim 1.18$ and vanishing for $a\gtrsim 1.18$ for certain ranges of inclination angles. We further analyze the effective angular momentum of photon orbits to demonstrate the fundamental connections between light geodesics and the KNS shadow geometry. We also perform numerical general relativistic ray tracing calculations, which reproduce the analytical topological change in the KNS shadow and illustrate other observational features within the shadow due to the lack of an event horizon. By comparing with black hole shadow observations, the topological change in the shadow of KNSs can be used to test the cosmic censorship hypothesis and KNSs as alternative models to SMBHs.

arXiv comment13 pages, 10 figures, accepted for publication in ApJ

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• Light rings and shadows of static black holes in effective quantum gravity II: A new solution without Cauchy horizons
• Modelling the Sgr A${}^{\ast }$ and M87${}^{\ast }$ shadows by using the Kerr-Taub-NUT metrics in the presence of a scalar field
• Gravitational field and lensing of a circular chiral vorton
• Shadow geometry of Kerr MOG naked singularity and analysis of accretion disk luminosity
• Imaging the destruction of a rotating regular black hole
• Shadows cast by a class of rotating black bounces with an anisotropic fluid
• Shadow of rotating black holes with consistent thermodynamics
• Photon Ring Dimming as a Signature of Photon-Axion Conversion in Janis-Newman-Winicour Naked Singularity
• Shadows of naked singularities and superspinars related to the revisited Kerr-de Sitter spacetimes