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

TL;DR

This paper proposes a novel interferometric method to determine the direction and angular velocity of accretion flows around black holes, using lagged covariance analysis, with potential application to EHT observations of Sagittarius A*.

Contribution

It introduces a new technique using lagged covariance in interferometry to measure flow direction and velocity in turbulent black hole accretion disks, validated with GRMHD simulations.

Findings

Lagged covariance peaks reveal flow direction and angular velocity.

Method robustly estimates flow direction even at moderate inclinations.

Potential to improve black hole imaging and break inclination degeneracies.

Abstract

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