  Proxima b is a terrestrial-mass planet in the habitable-zone of Proxima
Centauri. Proxima Centauri's high stellar activity however casts doubt on the
habitability of Proxima b: sufficiently bright and frequent flares and any
associated proton events may destroy the planet's ozone layer, allowing lethal
levels of UV flux to reach its surface. In March 2016, the Evryscope observed
the first naked-eye-brightness superflare detected from Proxima Centauri.
Proxima increased in optical flux by a factor of ~68 during the superflare and
released a bolometric energy of 10^33.5 erg, ~10X larger than any
previously-detected flare from Proxima. Over the last two years the Evryscope
has recorded 23 other large Proxima flares ranging in bolometric energy from
10^30.6 erg to 10^32.4 erg; coupling those rates with the single superflare
detection, we predict at least five superflares occur each year. Simultaneous
high-resolution HARPS spectroscopy during the Evryscope superflare constrains
the superflare's UV spectrum and any associated coronal mass ejections. We use
these results and the Evryscope flare rates to model the photochemical effects
of NOx atmospheric species generated by particle events from this extreme
stellar activity, and show that the repeated flaring may be sufficient to
reduce the ozone of an Earth-like atmosphere by 90% within five years; complete
depletion may occur within several hundred kyr. The UV light produced by the
Evryscope superflare would therefore have reached the surface with ~100X the
intensity required to kill simple UV-hardy microorganisms, suggesting that life
would have to undergo extreme adaptations to survive in the surface areas of
Proxima b exposed to these flares.
