2018/02/07 by Pedro V. P. Cunha, Carlos A. R. Herdeiro, Carlos Herdeiro +2 · 1 citation
Mathematics · Physics and Astronomy · #Apparent horizon #Astronomy #Astrophysical Phenomena and Observations #Astrophysics #Astrophysics and Cosmic Phenomena #Black hole (networking) #Conical surface #Event horizon #Geometry #Gravitational singularity #Horizon #Mathematics #Naked singularity #Observable #Optics #Photon #Photon sphere #Physics #Pulsars and Gravitational Waves Research #Quantum mechanics #Shadow (psychology) #Singularity #White hole #astro-ph.HE #gr-qc #hep-th
paper · pdf · doi:10.1103/physrevd.97.084020
published as Phys. Rev. D 97, 084020 (2018) · 15 pages, 9 figures
arxiv created 2018/02/07 · openalex publication_date 2018/04/16 · arxiv updated 2018/04/25 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06
There is an exciting prospect of obtaining the shadow of astrophysical black holes (BHs) in the near future with the Event Horizon Telescope. As a matter of principle, this justifies asking how much one can learn about the BH horizon itself from such a measurement. Since the shadow is determined by a set of special photon orbits, rather than horizon properties, it is possible that different horizon geometries yield similar shadows. One may then ask how sensitive is the shadow to details of the horizon geometry? As a case study, we consider the double Schwarzschild BH and analyze the impact on the lensing and shadows of the conical singularity that holds the two BHs in equilibrium---herein taken to be a strut along the symmetry axis in between the two BHs. Whereas the conical singularity induces a discontinuity of the scattering angle of photons, clearly visible in the lensing patterns along the direction of the strut's location, it produces no observable effect on the shadows, whose edges remain everywhere smooth. The latter feature is illustrated by examples including both equal and unequal mass BHs. This smoothness contrasts with the intrinsic geometry of the (spatial sections of the) horizon of these BHs, which is not smooth, and provides a sharp example on how BH shadows are insensitive to some horizon geometry details. This observation, moreover, suggests that for the study of their shadows, this static double BH system may be an informative proxy for a dynamical binary.