Unprecedented variability of Sgr A* in NIR
  The electromagnetic counterpart to the Galactic center supermassive black
hole, Sgr A*, has been observed in the near-infrared for over 20 years and is
known to be highly variable. We report new Keck Telescope observations showing
that Sgr A* reached much brighter flux levels in 2019 than ever measured at
near-infrared wavelengths. In the K$^\prime$ band, Sgr A* reached flux levels
of $\sim6$ mJy, twice the level of the previously observed peak flux from
$>13,000$ measurements over 130 nights with the VLT and Keck Telescopes. We
also observe a factor of 75 change in flux over a 2-hour time span with no
obvious color changes between 1.6 $\mu$m and 2.1 $\mu$m. The distribution of
flux variations observed this year is also significantly different than the
historical distribution. Using the most comprehensive statistical model
published, the probability of a single night exhibiting peak flux levels
observed this year, given historical Keck observations, is less than $0.3\%$.
The probability to observe the flux levels similar to all 4 nights of data in
2019 is less than $0.05\%$. This increase in brightness and variability may
indicate a period of heightened activity from Sgr A* or a change in its
accretion state. It may also indicate that the current model is not sufficient
to model Sgr A* at high flux levels and should be updated. Potential physical
origins of Sgr A*'s unprecedented brightness may be from changes in the
accretion-flow as a result of the star S0-2's closest passage to the black hole
in 2018 or from a delayed reaction to the approach of the dusty object G2 in
2014. Additional multi-wavelength observations will be necessary to both
monitor Sgr A* for potential state changes and to constrain the physical
processes responsible for its current variability.
