The Effects of Accretion Flow Dynamics on the Black Hole Shadow of Sagittarius A$^{*}$

TL;DR

This paper explores how different accretion flow dynamics around Sagittarius A* influence its observable shadow features, emphasizing the importance of flow models in interpreting EHT observations.

Contribution

It compares three distinct RIAF models with different flow dynamics and demonstrates their impact on shadow images and spectra, aiding interpretation of EHT data.

Findings

Flow dynamics significantly affect shadow images and spectra.

Structural variations in Sgr A* can be explained by non-stationary RIAF dynamics.

Flow energy shifts are crucial for interpreting mm/submm VLBI observations.

Abstract

A radiatively inefficient accretion flow (RIAF), which is commonly characterized by its sub-Keplerian nature, is a favored accretion model for the supermassive black hole at Galactic center, Sagittarius A$^{*}$. To investigate the observable features of a RIAF, we compare the modeled shadow images, visibilities, and spectra of three flow models with dynamics characterized by (i) a Keplerian shell which is rigidly-rotating outside the innermost stable circular orbit (ISCO) and infalling with a constant angular momentum inside ISCO, (ii) a sub-Keplerian motion, and (iii) a free-falling motion with zero angular momentum at infinity. At near-mm wavelengths the emission is dominated by the flow within several Schwarzschild radii. The energy shift due to the flow dynamics becomes important and distinguishable, suggesting that the flow dynamics are an important model parameter for interpreting the mm/submillimeter very long baseline interferometric observations with the forthcoming, fully assembled Event Horizon Telescope (EHT). As an example, we demonstrate that structural variations of Sagittarius A$^{*}$ on event horizon-scales detected in previous EHT observations can be explained by the non-stationary dynamics of a RIAF.