@article {1642261, title = {A Universal Power-law Prescription for Variability from Synthetic Images of Black Hole Accretion Flows }, journal = {The Astrophysical Journal Letters}, volume = {930}, number = {L20}, year = {2022}, pages = {1-32}, abstract = { We present a framework for characterizing the spatiotemporal power spectrum of the variability expected from the horizon-scale emission structure around supermassive black holes, and we apply this framework to a library of general relativistic magnetohydrodynamic (GRMHD) simulations and associated general relativistic ray-traced images relevant for Event Horizon Telescope (EHT) observations of Sgr A*. We find that the variability power spectrum is generically a red-noise process in both the temporal and spatial dimensions, with the peak in power occurring on the longest timescales and largest spatial scales. When both the time-averaged source structure and the spatially integrated light-curve variability are removed, the residual power spectrum exhibits a universal broken power-law behavior. On small spatial frequencies, the residual power spectrum rises as the square of the spatial frequency and is proportional to the variance in the centroid of emission. Beyond some peak in variability power, the residual power spectrum falls as that of the time-averaged source structure, which is similar across simulations; this behavior can be naturally explained if the variability arises from a multiplicative random field that has a steeper high-frequency power-law index than that of the time-averaged source structure. We briefly explore the ability of power spectral variability studies to constrain physical parameters relevant for the GRMHD simulations, which can be scaled to provide predictions for black holes in a range of systems in the optically thin regime. We present specific expectations for the behavior of the M87* and Sgr A* accretion flows as observed by the EHT. }, url = {https://doi.org/10.3847/2041-8213/ac65ebGeorgiev, Boris }, author = {Georgiev, Boris and Pesce, Dominic W. and Broderick, Avery E. and Dhruv, Vedant and Gammie, Charles F. and Chan, Chi-Kwan and Chatterjee, Koushik and Razieh Emami and Mizuno, Yosuke and Gold, Roman and Fromm, Christian M. and Ricarte, Angelo and Yoon, Doosoo and Joshi, Abhishek V. and Prather, Ben and Cruz-Osorio, Alejandro and Johnson, Michael D. and Porth, Oliver and Olivares, H{\'e}ctor and Younsi, Ziri and Luciano Rezzolla and Vos, Jesse and Qiu, Richard and Nathanail, Antonios and Ramesh Narayan and Chael, Andrew and Richard Anantua and Moscibrodzka, Monika and The Event Horizon Telescope Collaboration} }