Detection of a Millimeter Flare from Proxima Centauri

We present new analyses of ALMA 12 m and Atacama Compact Array (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 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 +/- 4 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 proportional to nu(alpha) with alpha = -1.77 +/- 0.45, and a lower limit on the fractional linear polarization of vertical bar Q/I vertical bar = 0.19 +/- 0.02. Because 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 +/- 9 mu Jy observed in the 12 m observations, compared to the photospheric flux density of 74 +/- 4 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 similar to 0.4 au is also removed.