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 Cycle 5 observations. Measuring the flux density of Sgr A∗ in 1 minute snapshots at 217.5, 219.5, and 234.0 GHz, we obtained light curves for ten 70 minute 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 × 10⁶ M 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.