Brightness Asymmetry of Black Hole Images as a Probe of Observer Inclination

Abstract

The Event Horizon Telescope recently captured images of the supermassive black hole in the center of the M87 galaxy, which show a ring-like emission structure with the South side only slightly brighter than the North side. This relatively weak asymmetry in the brightness profile along the ring has been interpreted as a consequence of the low inclination of the observer (around 17 deg for M87), which suppresses the Doppler beaming and boosting effects that might otherwise be expected due to the nearly relativistic velocities of the orbiting plasma. In this work, we use a large suite of general relativistic magnetohydrodynamic simulations to reassess the validity of this argument. By constructing explicit counter examples, we show that low-inclination is a sufficient but not necessary condition for images to have low brightness asymmetry. Accretion flow models with high accumulated magnetic flux close to the black hole horizon (the so-called magnetically arrested disks) and low black-hole spins have angular velocities that are substantially smaller than the orbital velocities of test particles at the same location. As a result, such models can produce images with low brightness asymmetry even when viewed edge on.

arXiv comment10 pages, 11 figures, submitted to ApJ

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• Using Machine Learning to Link Black Hole Accretion Flows with Spatially Resolved Polarimetric Observables
• Near-Horizon Polarization as a Diagnostic of Black Hole Spacetime
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• Principal-Component Interferometric Modeling (PRIMO), an Algorithm for EHT Data I: Reconstructing Images from Simulated EHT Observations
• New estimates of the spin and accretion rate of the black hole M87*
• A Red-Noise Eigenbasis for the Reconstruction of Blobby Images
• Observational distinguishability of the Kerr and Kerr-Hayward metrics to EHT