Does the black hole shadow probe the event horizon geometry?

Pedro V. P. Cunha, Carlos A. R. Herdeiro, and Maria J. Rodriguez
Phys. Rev. D 97, 084020 – Published 16 April 2018

Abstract

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.

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  • Received 15 February 2018

DOI:https://doi.org/10.1103/PhysRevD.97.084020

© 2018 American Physical Society

Physics Subject Headings (PhySH)

Gravitation, Cosmology & Astrophysics

Authors & Affiliations

Pedro V. P. Cunha1,2, Carlos A. R. Herdeiro1, and Maria J. Rodriguez3,4

  • 1Departamento de Física da Universidade de Aveiro and Centre for Research and Development in Mathematics and Applications (CIDMA), Campus de Santiago, 3810-183 Aveiro, Portugal
  • 2Centro de Astrofísica e Gravitação-CENTRA, Departamento de Física, Instituto Superior Técnico-IST, Universidade de Lisboa-UL, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal
  • 3Max Planck for Gravitational Physics-Albert Einstein Institute, Am Mühlenberg 1, Potsdam 14476, Germany
  • 4Department of Physics, Utah State University, 4415 Old Main Hill, Utah 84322, USA

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Issue

Vol. 97, Iss. 8 — 15 April 2018

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