Visibility of Black Hole Shadows in Low-luminosity AGN

Abstract

Accreting black holes tend to display a characteristic dark central region called the black-hole shadow, which depends only on spacetime/observer geometry and which conveys information about the black hole's mass and spin. Conversely, the observed central brightness depression, or image shadow, additionally depends on the morphology of the emission region. In this paper, we investigate the astrophysical requirements for observing a meaningful black-hole shadow in GRMHD-based models of accreting black holes. In particular, we identify two processes by which the image shadow can differ from the black-hole shadow: evacuation of the innermost region of the accretion flow, which can render the image shadow larger than the black-hole shadow, and obscuration of the black-hole shadow by optically thick regions of the accretion flow, which can render the image shadow smaller than the black-hole shadow, or eliminate it altogether. We investigate in which models the image shadows of our models match their corresponding black-hole shadows, and in which models the two deviate from each other. We find that, given a compact and optically thin emission region, our models allow for measurement of the black-hole shadow size to an accuracy of 5%. We show that these conditions are generally met for all MAD simulations we considered, as well as some of the SANE simulations.

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Cited by in corpus (16)

• Observing the Inner Shadow of a Black Hole: A Direct View of the Event Horizon
• No-Hair Theorem in the Wake of Event Horizon Telescope
• Photon rings of spherically symmetric black holes and robust tests of non-Kerr metrics
• Analytical study of higher-order ring images of accretion disk around black hole
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• Is a black hole shadow a reliable test of the no-hair theorem?
• Spacetime Measurements with the Photon Ring
• Magnetic flux eruptions at the root of time-lags in low-luminosity AGN
• The Nature of Black Hole Shadows