The Inner Shadow of the Black Hole in M87*: A Direct View of the Event Horizon

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”. The photon ring tracks light rays that asymptote to unstably bound photon orbits around the black hole; its size and shape are purely governed by the Kerr geometry. In contrast, the size, shape, and depth of the central brightness depression all depend on the details of the emission region. Images of spherical accretion models have a central brightness depression — the “black hole shadow” –- that completely fills the photon ring. By contrast, in models of equatorial disks extending to the black hole’s event horizon, the darkest region in the image is restricted to a much smaller area – an “inner shadow” – whose edge lies near the directly lensed image of the equatorial horizon. Using both semi-analytic models and general relativistic magnetohydrodynamic simulations, we demonstrate that the photon ring and inner shadow may be simultaneously visible in submillimeter images of M87*, where favored magnetically arrested disk models predict that the emission arises in a thin region near the equatorial plane. We show that the relative size, shape, and centroid of the photon ring and inner shadow can be used to estimate the black hole mass and spin, breaking degeneracies in measurements of these quantities from a single image feature. We show that the inner shadow in M87* may be visible in high-dynamic-range images produced by a next-generation Event Horizon Telescope.