Andrew Chael (Princeton University)

Simulated images of a black hole surrounded by optically thin emission typically display two main features: acentral brightness depression and a narrow, bright “photon ring” consisting of strongly lensed images superposedon top of the direct emission. The photon ring closely tracks a theoretical curve on the image plane correspondingto light rays that asymptote to unstably bound photon orbits around the black hole. This critical curve has asize and shape that are purely governed by the Kerr geometry; in contrast, the size, shape, and depth of theobserved brightness depression all depend on the details of the emission region. For instance, images of sphericalaccretion models display a distinctive dark region—the “black hole shadow”—that completely fills the photonring. By contrast, in models of equatorial disks extending to the black hole’s event horizon, the darkest regionin the image is restricted to a much smaller area—aninner shadow—whose edge lies near the direct lensedimage of the equatorial horizon. Using both semi-analytic models and general relativistic magnetohydrodynamic(GRMHD) simulations, we demonstrate that the photon ring and inner shadow may be simultaneously visible insubmillimeter images of M87, where magnetically arrested disk (MAD) simulations predict that the emissionarises in a thin region near the equatorial plane. We show that the relative size, shape, and centroid of thephoton ring and inner shadow can be used to estimate the black hole mass and spin, breaking degeneracies inmeasurements of these quantities that rely on the photon ring alone. Both features may be accessible to directobservation via high-dynamic-range images with a next-generation Event Horizon Telescope.

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