First Sagittarius A* Event Horizon Telescope Results. VI: Testing the Black Hole Metric

Abstract

Astrophysical black holes are expected to be described by the Kerr metric. This is the only stationary, vacuum, axisymmetric metric, without electromagnetic charge, that satisfies Einstein's equations and does not have pathologies outside of the event horizon. We present new constraints on potential deviations from the Kerr prediction based on 2017 EHT observations of Sagittarius A* (Sgr A*). We calibrate the relationship between the geometrically defined black hole shadow and the observed size of the ring-like images using a library that includes both Kerr and non-Kerr simulations. We use the exquisite prior constraints on the mass-to-distance ratio for Sgr A* to show that the observed image size is within $\sim$ 10$\%$ of the Kerr predictions. We use these bounds to constrain metrics that are parametrically different from Kerr as well as the charges of several known spacetimes. To consider alternatives to the presence of an event horizon we explore the possibility that Sgr A* is a compact object with a surface that either absorbs and thermally re-emits incident radiation or partially reflects it. Using the observed image size and the broadband spectrum of Sgr A*, we conclude that a thermal surface can be ruled out and a fully reflective one is unlikely. We compare our results to the broader landscape of gravitational tests. Together with the bounds found for stellar mass black holes and the M87 black hole, our observations provide further support that the external spacetimes of all black holes are described by the Kerr metric, independent of their mass.

55 pages, 27 figures, published in The Astrophysical Journal Letters on May 12, 2022. See the published paper for the full authors list

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• Optical appearance of regularized compact objects without an exterior photon sphere
• Macroscopic backreaction of the trace anomaly on classical vacuum backgrounds
• Scattering of massive particles from black holes and naked singularities
• Testing General Relativity with Black Hole X-Ray Data and ABHModels
• Comment on "Strong lensing and shadow of Ayon-Beato-Garcia (ABG) nonsingular black hole"
• Multiobjective optimization for scattering mitigation and scattering screen reconstruction in VLBI observations of the Galactic Center
• Investigating the interplay of the braneworld gravity and the plasma environment on the black hole shadow
• Constraints on horizonless objects after the EHT observation of Sagittarius A*
• Multi-ring images of thin accretion disk of a regular naked compact object
• Electromagnetic emission from a black-to-white hole transition -- Photospheric emission
• Shadows of generalised Hayward spacetimes : in vacuum and with plasma
• Improved Constraints on Non-Kerr Deviations from Binary Black Hole Inspirals Using GWTC-4 Data
• Images of the accretion disk in Hybrid metric-Palatini gravity
• Relativistic Thermal Emission from Accretion Disks in Kerr-MOG Spacetimes
• Thin accretion disks around a Schwarzschild acoustic black hole
• Testing the black hole area law with Event Horizon Telescope
• Observational distinguishability of the Kerr and Kerr-Hayward metrics to EHT
• Shaving off soft hairs and the black hole image memory effect
• Parameter Inference of Black Hole Images using Deep Learning in Visibility Space
• (In)equivalence of Metric-Affine and Metric Effective Field Theories
• Sagittarius A* near-infrared flares polarization as a probe of space-time I: Non-rotating exotic compact objects
• Gravitational wave memory in wormhole spacetimes
• Testing General Relativity with Black Holes
• Scattering from compact objects: Debye series and Regge-Debye poles
• Universality of the Blandford-Znajek emission in stationary and axisymmetric spacetimes