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

The black hole in the center of the Galaxy, associated with the compactsource Sagittarius A* (Sgr A*), is predicted to cast a shadow upon theemission of the surrounding plasma flow, which encodes the influence ofgeneral relativity (GR) in the strong-field regime. The Event HorizonTelescope (EHT) is a Very Long Baseline Interferometry (VLBI) networkwith a goal of imaging nearby supermassive black holes (in particularSgr A* and M87) with angular resolution sufficient to observe stronggravity effects near the event horizon. General relativisticmagnetohydrodynamic (GRMHD) simulations show that radio emission fromSgr A* exhibits variability on timescales of minutes, much shorter thanthe duration of a typical VLBI imaging experiment, which usually takesseveral hours. A changing source structure during the observations,however, violates one of the basic assumptions needed for aperturesynthesis in radio interferometry imaging to work. By simulatingrealistic EHT observations of a model movie of Sgr A*, we demonstratethat an image of the average quiescent emission, featuring thecharacteristic black hole shadow and photon ring predicted by GR, cannonetheless be obtained by observing over multiple days and subsequentprocessing of the visibilities (scaling, averaging, and smoothing)before imaging. Moreover, it is shown that this procedure can becombined with an existing method to mitigate the effects of interstellarscattering. Taken together, these techniques allow the black hole shadowin the Galactic center to be recovered on the reconstructed image.