Probing the Innermost Accretion Flow Geometry of Sgr A* with Event Horizon Telescope

Abstract

Upcoming Event Horizon Telescope (EHT) observations will provide a unique opportunity to reveal the innermost region of the radiative inefficient accretion flow (RIAF) around the Galactic black hole, Sgr A*. Depending on the flow dynamics and accumulated magnetic flux, the innermost region of an RIAF could have a quasi-spherical or disk-like geometry. Here we present a phenomenological model to investigate the characteristics of the black hole shadow images with different flow geometries, together with the effect of black hole spin and flow dynamics. The resulting image consists in general of two major components: a crescent, which may surround the funnel region of the black hole or the black hole itself, and a photon ring, which may be partially luminous and overlapped with the crescent component. Compared to a quasi-spherical flow case, a disk-like flow in the vicinity of a black hole exhibits the following image features: (i) due to less material near the funnel region, the crescent structure has a smaller size, and (ii) due to the combination of emission from the flow beside and behind the black hole, the crescent structure has a more irregular shape, and a less smooth brightness distribution. How these features can result in different observables for EHT observations is discussed.