Testing horizon topology with electromagnetic observations

Abstract

In general relativity without a cosmological constant, a classical theorem due to Hawking states that stationary black holes must be topologically spherical. This result is one of the several ingredients that collectively imply the uniqueness of the Kerr metric. If, however, general relativity describes gravity inexactly at high energies or over cosmological scales, Hawking's result may not apply, and black holes with non-trivial topology may be, at least mathematically, permissible. While tests involving electromagnetic and gravitational-wave data have been used to place tight constraints on various theoretical departures from a Kerr description of astrophysical black holes, relatively little attention has been paid to topological alternatives. In this paper, we derive a new exact solution in an $f(R)$ theory of gravity which admits topologically non-trivial black holes, and calculate observables like fluorescent K$\alpha$ iron-line profiles and black hole images from hypothetical astrophysical systems which house these objects, to provide a theoretical basis for new tests of black hole nature. On the basis of qualitative comparisons, we show that topologically non-trivial objects would leave a strong imprint on electromagnetic observables and can be easily distinguished from general-relativistic black holes in nearly all cases.

15 pages, 9 figures. Comments welcome

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• Axion-plasmon or magnetized plasma effect on an observable shadow and gravitational lensing of a Schwarzschild black hole
• Quantum effects on the black hole shadow and deflection angle in presence of plasma
• Black holes surrounded by Einstein clusters as models of dark matter fluid
• Effect of magnetized plasma on shadow and gravitational lensing of a Reissner-Nordström black hole
• Exact theory for the Rezzolla-Zhidenko metric and self-consistent calculation of quasinormal modes
• Regular solutions for black strings and torus-like black holes
• Premerger phenomena in neutron-star binary coalescences
• General relativistic radiation transport: Implications for VLBI/EHT observations of AGN discs, winds and jets