Measuring the Direction and Angular Velocity of a Black Hole Accretion Disk via Lagged Interferometric Covariance

We show that interferometry can be applied to study irregular, rapidlyrotating structures, as are expected in the turbulent accretion flownear a black hole. Specifically, we analyze the lagged covariancebetween interferometric baselines of similar lengths but slightlydifferent orientations. For a flow viewed close to face-on, wedemonstrate that the peak in the lagged covariance indicates thedirection and angular velocity of the emission pattern from the flow.Even for moderately inclined flows, the covariance robustly estimatesthe flow direction, although the estimated angular velocity can besignificantly biased. Importantly, measuring the direction of the flowas clockwise or counterclockwise on the sky breaks a degeneracy inaccretion disk inclinations when analyzing time-averaged images alone.We explore the potential efficacy of our technique usingthree-dimensional, general relativistic magnetohydrodynamic simulations,and we highlight several baseline pairs for the Event Horizon Telescope(EHT) that are well-suited to this application. These results indicatethat the EHT may be capable of estimating the direction and angularvelocity of the emitting material near Sgr A*, and they suggest that arotating flow may even be utilized to improve imaging capabilities.