A Universal Power-law Prescription for Variability from Synthetic Images of Black Hole Accretion Flows

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.

Superradiant evolution of the shadow and photon ring of Sgr A$^\star$

“…Because of the short dynamical timescale ∼ 20 s, the EHT has pushed VLBI techniques to image the dynamical evolution of Sgr A by reconstructing the source's emission region [18,19]. Assuming General Relativity, the estimated dynamical mass and distance are consistent with the angular diameter of the shadow (51.8 ± 2.3) µas recently reported by the EHT collaboration [20][21][22][23][24][25][26][27][28][29].…”

First Sagittarius A* Event Horizon Telescope Results. V. Testing Astrophysical Models of the Galactic Center Black Hole

“…The measured s var 2 is well characterized by a power law for 2 Gλ < |u| < 6 Gλ (Georgiev et al 2022). For comparison with the models presented here, we distill the s var 2 to two numbers: the amplitude a Model predictions for a 4 2 and b are computed using the power spectral densities from Georgiev et al (2022). 160 The anisotropic diffractive scattering kernel from Johnson et al (2018) is applied to…”

First Sagittarius A* Event Horizon Telescope Results. IV. Variability, Morphology, and Black Hole Mass

“…This behavior is generic across a large number of simulations of Sgr A * and explored in detail inGeorgiev et al (2022).…”