  We present new analyses of ALMA 12-m and ACA observations at 233 GHz (1.3 mm)
of the Proxima Centauri system with sensitivities of 9.5 and 47 $\mu$Jy
beam$^{-1}$, respectively, taken from 2017 January 21 through 2017 April 25.
These analyses reveal that the star underwent a significant flaring event
during one of the ACA observations on 2017 March 24. The complete event lasted
for approximately 1 minute and reached a peak flux density of $100\pm4$ mJy,
nearly a factor of $1000\times$ brighter than the star's quiescent emission. At
the flare peak, the continuum emission is characterized by a steeply falling
spectral index with frequency, $F_\nu \propto \nu^\alpha$ with $\alpha =
-1.77\pm0.45$, and a lower limit on the fractional linear polarization of
$|Q/I| = 0.19\pm0.02$. Since the ACA observations do not show any quiescent
excess emission, we conclude that there is no need to invoke the presence of a
dust belt at $1-4$ AU. We also posit that the slight excess flux density of
$101\pm9$ $\mu$Jy observed in the 12-m observations compared to the
photospheric flux density of $74\pm4$ $\mu$Jy extrapolated from infrared
wavelengths may be due to coronal heating from continual smaller flares, as is
seen for AU Mic, another nearby, well-studied, M dwarf flare star. If this is
true, then the need for warm dust at $\sim0.4$ AU is also removed.
