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 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 the light curves which are
simultaneously of high time resolution (2 seconds) 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 June 2019 variability behavior with that of 2001-2017, and suggest that the
most likely cause of the bright NIR flares is magnetic reconnection.
