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

Abstract

Abstract Simulated images of a black hole surrounded by optically thin emission typically display two main features: a central brightness depression and a narrow “photon ring” consisting of strongly lensed images superimposed over the direct emission. The photon ring closely tracks a theoretical curve on the image plane corresponding to light rays that asymptote to bound photon orbits. The size and shape of this critical curve are purely governed by the Kerr geometry; in contrast, the size, shape, and depth of the observed brightness depression 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. By contrast, in models of equatorial disks extending to the event horizon, the darkest region in the image is restricted to a much smaller area—an inner shadow—whose edge lies near the direct lensed image of the equatorial horizon. Using both general relativistic MHD simulations and semi-analytic models, we demonstrate that the photon ring and inner shadow may be simultaneously visible in submillimeter images of M87*, where magnetically arrested disk simulations 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 the photon ring alone. Both features may be accessible to direct observation via high-dynamic-range images with a next-generation Event Horizon Telescope.