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The Event Horizon Telescope (EHT) observed the compact radio source,Sagittarius A* (Sgr A*), in the Galactic Center on 2017 April 5-11 in the 1.3millimeter wavelength band. At the same time, interferometric array data fromthe Atacama Large Millimeter/submillimeter Array and the Submillimeter Arraywere collected, providing Sgr A* light curves simultaneous with the EHTobservations. These data sets, complementing the EHT very-long-baselineinterferometry, are characterized by a cadence and signal-to-noise ratiopreviously unattainable for Sgr A* at millimeter wavelengths, and they allowfor the investigation of source variability on timescales as short as a minute.While most of the light curves correspond to a low variability state of Sgr A*,the April 11 observations follow an X-ray flare, and exhibit strongly enhancedvariability. All of the light curves are consistent with a red noise process,with a power spectral density (PSD) slope measured to be between -2 and -3 ontimescales between 1 min and several hours. Our results indicate a steepeningof the PSD slope for timescales shorter than 0.3 h. The spectral energydistribution is flat at 220 GHz and there are no time-lags between the 213 and229 GHz frequency bands, suggesting low optical depth for the event horizonscale source. We characterize Sgr A*'s variability, highlighting the differentbehavior observed just after the X-ray flare, and use Gaussian process modelingto extract a decorrelation timescale and a PSD slope. We also investigate thesystematic calibration uncertainties by analyzing data from independent datareduction pipelines.

Millimeter light curves of Sagittarius A* observed during the 2017 Event Horizon Telescope campaign