Observing the Inner Shadow of a Black Hole: A Direct View of the Event Horizon

Understand

Simulated images of a black hole surrounded by optically thin emission typically display two main features: a central brightness depression and a narrow, bright "photon ring" consisting of strongly lensed images superposed on top of the direct emission.

• The photon ring closely tracks a theoretical curve on the image plane corresponding to light rays that asymptote to unstably bound photon orbits around the black hole.
• This critical curve has a size and shape that are purely governed by the Kerr geometry; in contrast, the size, shape, and depth of the observed brightness depression all depend on the details of the emission region.
• For instance, images of spherical accretion models display a distinctive dark region -- the "black hole shadow" -- that completely fills the photon ring.