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

TL;DR

This study challenges the assumption that low brightness asymmetry in black hole images indicates low observer inclination, showing that certain accretion models can produce symmetric images even at high inclinations.

Contribution

It demonstrates that low brightness asymmetry is not solely due to low inclination, using simulations of magnetically arrested disks with various spins.

Findings

Low brightness asymmetry can occur at high inclinations in certain accretion models.

Magnetically arrested disks with low spins produce less angular velocity, reducing Doppler effects.

Brightness asymmetry is not a definitive indicator 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.