Imaging an Event Horizon: Mitigation of Source Variability of Sagittarius A*

TL;DR

This paper demonstrates that by averaging and smoothing VLBI data over multiple days, the black hole shadow of Sagittarius A* can be effectively imaged despite rapid source variability and interstellar scattering effects.

Contribution

It introduces a method combining multi-day averaging and scattering mitigation to recover the black hole shadow in VLBI images of Sgr A* despite short-term variability.

Findings

Black hole shadow can be recovered through multi-day data averaging.

Source variability can be mitigated with data processing techniques.

Interstellar scattering effects can be reduced alongside variability mitigation.

Abstract

The black hole in the center of the Galaxy, associated with the compact source Sagittarius A* (Sgr A*), is predicted to cast a shadow upon the emission of the surrounding plasma flow, which encodes the influence of general relativity in the strong-field regime. The Event Horizon Telescope (EHT) is a Very Long Baseline Interferometry (VLBI) network with a goal of imaging nearby supermassive black holes (in particular Sgr A* and M87) with angular resolution sufficient to observe strong gravity effects near the event horizon. General relativistic magnetohydrodynamic (GRMHD) simulations show that radio emission from Sgr A* exhibits vari- ability on timescales of minutes, much shorter than the duration of a typical VLBI imaging experiment, which usually takes several hours. A changing source structure during the observations, however, violates one of the basic assumptions needed for aperture synthesis in radio interferometry imaging to work. By simulating realistic EHT observations of a model movie of Sgr A*, we demonstrate that an image of the average quiescent emission, featuring the characteristic black hole shadow and photon ring predicted by general relativity, can nonetheless be obtained by observing over multiple days and subsequent processing of the visibilities (scaling, averaging, and smoothing) before imaging. Moreover, it is shown that this procedure can be combined with an existing method to mitigate the effects of interstellar scattering. Taken together, these techniques allow the black hole shadow in the Galactic center to be recovered on the reconstructed image.