Time Variations in the Flux Density of Sgr A* at 230 GHz Detected with
  ALMA
  A radio source at the Galactic center Sgr A* is a prime supermassive black
hole candidate and therefore key to developing our understanding of them. Time
variations in the 230 GHz band flux of Sgr A* have been found with the Atacama
Large Millimeter/submillimeter Array (ALMA) Cycle 5 observations. Measuring the
flux density of Sgr A* in 1 min snapshots at 217.5, 219.5, and 234.0 GHz, we
obtained light curves for ten 70 min periods. The light curves show variations
at a few tens of minutes and hourly scales. The shorter timescale is similar to
the orbital period of the innermost stable circular orbit around a $4\times
10^{6}$ $M_{\odot}$ black hole, suggesting that the variation originates from
the immediate vicinity of Sgr A*. We also detected no time lag between 217.5
and 234.0 GHz and a dependence of the spectral index on the flux density.
