Imaging the Shadow of a Black Hole through Sgr A* Monitoring

We propose to monitor the near-IR flare emission of Sgr A* at two wavelengths using NIRCam imaging. The observations will consist of 2 12-hour episodes to be carried out simulataneously with observations by ALMA and the Event Horizon Telescope (EHT). Simultaneous measurements at 2.1 and 4.8 microns provide the spectral evolution of IR flare emission. This yields information about the particle acceleration process and subsequent synchrotron cooling of the highest-energy particles. Using an adiabatic expansion model, the IR light curve can then be used to determine the spectrum and the variability of submm emission across the multiple flares that are likely during the course of the EHT observations. The predicted submm variability is a necessary component in being able to construct an image of the black hole shadow from the EHT observations. The proposed simultaneous IR and submm observations of the variability of Sgr A* will be key to the EHT's success.