Journal ArticleRotation in Event Horizon Telescope MoviesConroy, Nicholas S.; Bauböck, Michi; Dhruv, Vedant; Lee, Daeyoung; Broderick, Avery E.; Chan, Chi-kwan; Georgiev, Boris; Joshi, Abhishek V.; Prather, Ben; Gammie, Charles F.<title>Abstract</title> <p>The Event Horizon Telescope (EHT) has produced images of M87* and Sagittarius A*, and will soon produce time sequences of images, or movies. In anticipation of this, we describe a technique to measure the rotation rate, or pattern speed Ω_{<italic>p</italic>}, from movies using an autocorrelation technique. We validate the technique on Gaussian random field models with a known rotation rate and apply it to a library of synthetic images of Sgr A* based on general relativistic magnetohydrodynamics simulations. We predict that EHT movies will have Ω_{<italic>p</italic>}≈ 1° per<italic>GMc</italic>⁻³, which is of order 15% of the Keplerian orbital frequency in the emitting region. We can plausibly attribute the slow rotation seen in our models to the pattern speed of inward-propagating spiral shocks. We also find that Ω_{<italic>p</italic>}depends strongly on inclination. Application of this technique will enable us to compare future EHT movies with the clockwise rotation of Sgr A* seen in near-infrared flares by GRAVITY. Pattern speed analysis of future EHT observations of M87* and Sgr A* may also provide novel constraints on black hole inclination and spin, as well as an independent measurement of black hole mass.</p>American Astronomical Society2023-07-0110474207The Astrophysical Journal9511460004-637Xhttps://doi.org/10.3847/1538-4357/acd2c81716327; 2034306National Science Foundation