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General relativistic magnetohydrodynamic (GRMHD) simulations of accretiondisks and jets associated with supermassive black holes show variability on awide range of timescales. On timescales comparable to or longer than thegravitational timescale $t_G=GM/c^3$, variation may be dominated by orbitaldynamics of the inhomogeneous accretion flow. Turbulent evolution within theaccretion disk is expected on timescales comparable to the orbital period,typically an order of magnitude larger than $t_G$. For Sgr A*, $t_G$ is muchshorter than the typical duration of a VLBI experiment, enabling us to studythis variability within a single observation. Closure phases, the sum ofinterferometric visibility phases on a triangle of baselines, are particularlyuseful for studying this variability. In addition to a changing sourcestructure, variations in observed closure phase can also be due to interstellarscattering, thermal noise, and the changing geometry of projected baselinesover time due to Earth rotation. We present a metric that is able todistinguish the latter two from intrinsic or scattering variability. Thismetric is validated using synthetic observations of GRMHD simulations of SgrA*. When applied to existing multi-epoch EHT data of Sgr A*, this metric showsthat the data are most consistent with source models containing intrinsicvariability from source dynamics, interstellar scattering, or a combination ofthose. The effects of black hole inclination, orientation, spin, and morphology(disk or jet) on the expected closure phase variability are also discussed.

Quantifying Intrinsic Variability of Sagittarius A* using Closure Phase Measurements of the Event Horizon Telescope