  We present a multiwavelength study of stellar flares on primarily G-type
stars using overlapping time domain surveys in the near ultraviolet (NUV) and
optical regimes. The NUV (GALEX) and optical (Kepler) wavelength domains are
important for understanding energy fractionations in stellar flares, and for
constraining the associated incident radiation on a planetary atmosphere. We
follow up on the NUV flare detections presented in Brasseur et al. 2019, using
coincident Kepler long (1557 flares) and short (2 flares) cadence light curves.
We find no evidence of optical flares at these times, and place limits on the
flare energy ratio between the two wavebands. We find that the energy ratio is
correlated with GALEX band energy, and extends over a range of about three
orders of magnitude in the ratio of the upper limit of Kepler band flare energy
to NUV flare energy at the same time for each flare. The two flares with Kepler
short cadence data indicate that the true Kepler band energy may be much lower
than the long cadence based upper limit. A similar trend appears for the bulk
flare energy properties of non-simultaneously observed flares on the same
stars. We provide updated models to describe the flare spectral energy
distribution from the NUV through the optical including continua and emission
lines to improve upon blackbody-only models. The spread of observed energy
ratios is much larger than encompassed by these models and suggests new physics
is at work. These results call for better understanding of NUV flare physics
and provide a cautionary tale about using only optical flare measurements to
infer the UV irradiation of close-in planets.
