Second-scale Submillimeter Variability of Sagittarius A* during Flaring Activity of 2019: On the Origin of Bright Near-infrared Flares

In 2019, Sgr A*-the supermassive black hole in the Galactic Center-underwent unprecedented flaring activity in the near-infrared (NIR), brightening by up to a factor of 100 compared to quiescent values. Here we report Atacama Large Millimeter/submillimeter Array (ALMA) observations of Sgr A*'s continuum variability at 1.3 mm (230 GHz)-a tracer of the accretion rate-conducted one month after the brightest detected NIR flare and in the middle of the flaring activity of 2019. We develop an innovative light-curve extraction technique which (together with ALMA's excellent sensitivity) allows us to obtain light curves that are simultaneously of high time resolution (2 s) and high signal-to-noise ratio (~500). We construct an accurate intrinsic structure function of the Sgr A* submm variability, improving on previous studies by about two orders of magnitude in timescale and one order of magnitude in sensitivity. We compare the 2019 June variability behavior with that of 2001-2017 and suggest that the most likely cause of the bright NIR flares is magnetic reconnection.