  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 1.3
millimeter wavelength band. At the same time, interferometric array data from
the Atacama Large Millimeter/submillimeter Array and the Submillimeter Array
were collected, providing Sgr A* light curves simultaneous with the EHT
observations. These data sets, complementing the EHT very-long-baseline
interferometry, are characterized by a cadence and signal-to-noise ratio
previously unattainable for Sgr A* at millimeter wavelengths, and they allow
for 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 enhanced
variability. 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 on
timescales between 1 min and several hours. Our results indicate a steepening
of the PSD slope for timescales shorter than 0.3 h. The spectral energy
distribution is flat at 220 GHz and there are no time-lags between the 213 and
229 GHz frequency bands, suggesting low optical depth for the event horizon
scale source. We characterize Sgr A*'s variability, highlighting the different
behavior observed just after the X-ray flare, and use Gaussian process modeling
to extract a decorrelation timescale and a PSD slope. We also investigate the
systematic calibration uncertainties by analyzing data from independent data
reduction pipelines.
