Visible shapes of black holes M87* and SgrA*

TL;DR

This paper reviews how visible images of supermassive black holes like M87* and SgrA* are formed, focusing on the differences between classical shadows and event horizon silhouettes, and relates observational data to theoretical models.

Contribution

It clarifies the physical origins of observable black hole features and interprets the EHT images in terms of accretion and shadow models, highlighting the visibility conditions of different black hole features.

Findings

The classical shadow appears with luminous backgrounds beyond photon spheres.

The event horizon silhouette is visible when accreting matter highlights the horizon.

EHT M87* image matches the event horizon silhouette size in certain accretion models.

Abstract

We review the physical origins for possible visible images of the supermassive black hole M87* in the galaxy M87 and SgrA* in the Milky Way Galaxy. The classical dark black hole shadow of the maximal size is visible in the case of luminous background behind the black hole at the distance exceeding the so-called photon spheres. The notably smaller dark shadow (dark silhouette) of the black hole event horizon is visible if the black hole is highlighted by the inner parts of the luminous accreting matter inside the photon spheres. The first image of the supermassive black hole M87 *, obtained by the Event Horizon Telescope collaboration, shows the lensed dark image of the southern hemisphere of the black hole event horizon globe, highlighted by accreting matter, while the classical black hole shadow is invisible at all. A size of the dark spot on the Event Horizon Telescope (EHT) image agrees with a corresponding size of the dark event horizon silhouette in a thin accretion disk model in the case of either the high or moderate value of the black hole spin, $a\gtrsim0.75$.