Characterizing and Mitigating Intraday Variability: Reconstructing Source Structure in Accreting Black Holes with mm-VLBI

Abstract

The extraordinary physical resolution afforded by the Event Horizon Telescope has opened a window onto the astrophysical phenomena unfolding on horizon scales in two known black holes, M87* and Sgr A*. However, with this leap in resolution has come a new set of practical complications. Sgr A* exhibits intraday variability that violates the assumptions underlying Earth aperture synthesis, limiting traditional image reconstruction methods to short timescales and data sets with very sparse (u, v) coverage. We present a new set of tools to detect and mitigate this variability. We develop a data-driven, model-agnostic procedure to detect and characterize the spatial structure of intraday variability. This method is calibrated against a large set of mock data sets, producing an empirical estimator of the spatial power spectrum of the brightness fluctuations. We present a novel Bayesian noise modeling algorithm that simultaneously reconstructs an average image and statistical measure of the fluctuations about it using a parameterized form for the excess variance in the complex visibilities not otherwise explained by the statistical errors. These methods are validated using a variety of simulated data, including general relativistic magnetohydrodynamic simulations appropriate for Sgr A* and M87*. We find that the reconstructed source structure and variability are robust to changes in the underlying image model. We apply these methods to the 2017 EHT observations of M87*, finding evidence for variability across the EHT observing campaign. The variability mitigation strategies presented are widely applicable to very long baseline interferometry observations of variable sources generally, for which they provide a data-informed averaging procedure and natural characterization of inter-epoch image consistency.

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

“…A measurement of the variability PSDs from EHT data is further obscured by instrumental and calibration effects. However, tests on synthetic data from GRMHD simulations in A. E. Broderick et al (2022) show that á ñ PT can be well constrained with current observations for 2 Gλ  |u|  6 Gλ.…”

“…In this work we characterize the spatiotemporal variability of the visibility domain representation of a broad collection of GRMHD simulations. We attempt to characterize the typical deviation of ( ) u v t , , from the average across the whole simulation, to provide a physically motivated prior for the measured source variability probed by the EHT (Broderick et al 2022). In order to facilitate the structural variability analysis of the resolved source, we consider the properties of simulated images normalized by the corresponding light curves (instantaneous total flux values).…”

“…Sgr A * and, to a lesser degree, M87 * necessarily violate the stationarity assumptions underlying Earth-aperture synthesis. In the case of the former, this presents a significant impediment to imaging and model comparison (Broderick et al 2022;Paper III;Paper IV).…”

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

“…To remove correlations associated with variations in the total flux, we normalize the VA data with the contemporaneous intrasite light curve (Georgiev et al 2022). The light-curve-normalized VAs are then linearly detrended, and variances are computed and azimuthally averaged (Broderick et al 2022). The resulting…”

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

“…To quantify the variability observed in the EHT Sgr A * data, we make use of the procedure described in Broderick et al (2022). This procedure provides an estimate of the excess variability-i.e., the visibility amplitude variance in excess of that caused by known sources, such as average source structure, statistical and systematic uncertainties, and scattering-as a function of baseline length.…”