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A radio source at the Galactic center Sgr A* is a prime supermassive blackhole candidate and therefore key to developing our understanding of them. Timevariations in the 230 GHz band flux of Sgr A* have been found with the AtacamaLarge Millimeter/submillimeter Array (ALMA) Cycle 5 observations. Measuring theflux density of Sgr A* in 1 min snapshots at 217.5, 219.5, and 234.0 GHz, weobtained light curves for ten 70 min periods. The light curves show variationsat a few tens of minutes and hourly scales. The shorter timescale is similar tothe orbital period of the innermost stable circular orbit around a $4\times10^{6}$ $M_{\odot}$ black hole, suggesting that the variation originates fromthe immediate vicinity of Sgr A*. We also detected no time lag between 217.5and 234.0 GHz and a dependence of the spectral index on the flux density.

Time Variations in the Flux Density of Sgr A* at 230 GHz Detected with ALMA