tokens: et al. 2018 ). In the X-ray, events up to 10 32 erg (10 32.7 erg bolometric) have been detected (Güdel et al. 2004 ). The Microvariability and Oscillation of Stars ( MOST ) satellite (Walker et al. 2003 ) performed the most comprehensive previous measurement of the Proxima flare rate. MOST observed Proxima for 37.6 days, observing 66 white-light flare events, the largest of which was 10 31.5 erg in the MOST bandpass (∼4500–7500 Å). No superflares were observed; extrapolating the cumulative flare frequency distribution (FFD) obtained by Davenport et al. ( 2016 ) from the MOST flare sample out by 1.5 dex predicts that ∼8 × 10 33+ erg events in the MOST bandpass occur per year. To search for superflares and other short-timescale phenomena, the Evryscope (Law et al. 2015 ) is performing long-term high-cadence monitoring of Proxima, along with every other bright star in the Southern sky. In 2016 March the Evryscope detected the first-known Proxima superflare. Although no M-dwarfs are usually visible to the naked eye (e.g., Shields et al. 2016 ), Proxima briefly became at least a magnitude-6.8 star during this superflare, at the limit of visibility to dark-site naked-eye observers. 2. Evryscope Flare Discovery and Observations We discovered the Proxima superflare as part of the Evryscope survey of all bright southern stars. The Evryscope is an array of small telescopes simultaneously imaging 8000 square degrees of the sky every two minutes (Law et al. 2015 ). The Evryscope observes essentially the entire Southern sky above an airmass of two, at two-minute cadence in g ′ and at a resolution of 13′′ pixel −1 . The system has a typical dark-sky limiting magnitude of g ′ = 16 and tracks the sky for 2 hr at a time before ratcheting back and continuing observations, for an average of ∼6 hr of continuous monitoring each night on each part of the sky. The Evryscope image archive contains 2.5 million raw images, which is ∼350TB of data. A custom pipeline analyzes the Evryscope data set at realtime rates (Law et al. 2016 ). Each image, consisting of a 30 MPix FITS file from one camera, is calibrated using a custom wide-field solver. After careful background modeling and subtraction, forced-aperture photometry is extracted based on known source positions in a reference catalog. Light curves are then generated for approximately 15 million sources across the Southern sky by differential photometry in small sky regions using carefully selected reference stars and a range of apertures; residual systematics are removed using two iterations of the SysRem detrending algorithm (Tamuz et al. 2005 ). In extremely crowded fields, such as for Proxima (−2° galactic latitude), we re-run the pipeline for particular targets, optimizing the aperture sizes to avoid nearby stars. As a very large event, the Proxima Superflare was discovered in routine by-eye checks of interesting targets in the Evryscope data set. Smaller flares are discovered and characterized with an automated
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