The EvryFlare Survey: High-cadence flare temperatures of a large sample of superflares and starspots observed simultaneously by Evryscope and TESS

The combined line and continuum emission of optical stellar flares is usually approximated by a 9000 K blackbody. The blackbody temperature is a key ingredient in modeling the effects of superflares upon the atmospheric photochemistry of Earth-like planets, governing the fraction of the stellar flare energy emitted at UV wavelengths. We observe hundreds of AD Leo type superflares from a diverse sample of K5-M5 stars with the Evryscope all sky array of small telescopes, including superflares from Proxima Cen and the LTT 1445 system. We measure flare blackbody temperatures using the color-evolution of dozens of superflares that have simultaneous 2 min cadence observations from both Evryscope and TESS. We test the assumption of a 9000 K blackbody against our superflares: preliminary results indicate some events reach in excess of 25,000 K. Planetary atmospheres and surface life may therefore potentially be exposed to an order-of-magnitude increase in UV radiation above that computed from a 9000 K blackbody. We explore flare properties as a function of stellar rotation. We observe a decrease in superflare rates and energies at longer stellar rotation periods, observing a possible change in the superflare rates of M-dwarfs at periods corresponding to the spin-down transition from quickly-rotating to slowly-rotating states. We also compare the amplitudes of rotation of each flare star in the Evryscope and TESS bands. We find the Evryscope amplitudes are larger than those in TESS; the effect is correlated with stellar mass. We measure a median starspot coverage of 13% of the stellar hemisphere and constrain the minimum magnetic field strength consistent with our flare energies and starspot coverage to be 500 G, with later-type stars exhibiting lower values than earlier-types.