Black Hole Images as Tests of General Relativity: Effects of Spacetime Geometry

Abstract

The images of supermassive black holes surrounded by optically-thin, radiatively-inefficient accretion flows, like those observed with the Event Horizon Telescope, are characterized by a bright ring of emission surrounding the black-hole shadow. In the Kerr spacetime this bright ring, when narrow, closely traces the boundary of the shadow and can, with appropriate calibration, serve as its proxy. The present paper expands the validity of this statement by considering two particular spacetime geometries: a solution to the field equations of a modified gravity theory and another that parametrically deviates from Kerr but recovers the Kerr spacetime when its deviation parameters vanish. A covariant, axisymmetric analytic model of the accretion flow based on conservation laws and spanning a broad range of plasma conditions is utilized to calculate synthetic non-Kerr black-hole images, which are then analysed and characterized. We find that in all spacetimes: (i) it is the gravitationally-lensed unstable photon orbit that plays the critical role in establishing the diameter of the rings observed in black-hole images, not the event horizon or the innermost stable circular orbit, (ii) bright rings in these images scale in size with, and encompass, the boundaries of the black-hole shadows, even when deviating significantly from Kerr, and (iii) uncertainties in the physical properties of the accreting plasma introduce subdominant corrections to the relation between the diameter of the image and the diameter of the black-hole shadow. These results provide important new theoretical justification for using black-hole images to probe and test the spacetimes of supermassive black holes.

30 pages, 18 figures, 2 tables. Accepted for publication in ApJ

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• Imaging ultra-compact objects with radiatively inefficient accretion flows
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• Determination of the spin parameter and the inclination angle by the relativistic images in black hole image
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• Impacts of nonthermal emission on the images of a black hole shadow and extended jets in two-temperature GRMHD simulations
• Explicit symplectic integrators with adaptive time steps in curved spacetimes
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• The ring-shaped shadow of rotating naked singularity with a complete photon sphere
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• Spacetime Measurements with the Photon Ring
• Expanding Sgr A* dynamical imaging capabilities with an African extension to the Event Horizon Telescope
• Deep learning inference with the Event Horizon Telescope III. Zingularity results from the 2017 observations and predictions for future array expansions
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• Imaging fermionic dark matter cores at the center of galaxies
• Fractal signatures of non-Kerr spacetimes in the shadow of light-ring bifurcations
• On the Universality of Energy Extraction from Black Hole Spacetimes
• Primary Proca Hair and the Double-Peak Optics of Black Holes
• Aberration effect on lower-order images of thin accretion disk in the astrometric approach
• Origin of the ring ellipticity in the black hole images of M87*
• Shadow of the Scalar Hairy Black Hole with Inverted Higgs Potential
• Not all spacetime coordinates for general-relativistic ray tracing are created equal
• Membrane Paradigm Approach to the Johannsen-Psaltis Black Hole
• Shadow of rotating black holes with consistent thermodynamics
• Observational distinguishability of the Kerr and Kerr-Hayward metrics to EHT
• Relativistic Thermal Emission from Accretion Disks in Kerr-MOG Spacetimes
• Impacts of radiative cooling on the images of a black hole shadow and extended jets in two-temperature GRMHD simulations
• Shadows and photon spheres of static black holes embedded in a Dehnen-(1,4,5/2)-type dark matter halo with a quintessential field