Visibility of Black Hole Shadows in Low-luminosity AGN

Understand

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.