2025AJ....170..249D

On 2017 September 20, we observed GJ 4334, an M5V dwarf rotating with a period of 23.5 days, simultaneously with both the Space Telescope Imaging Spectrograph aboard Hubble (1160-1710 Å) and the Dual Imaging Spectrograph mounted on the 3.5 m telescope at Apache Point Observatory (3750-5050; 5800-6950 Å) as part of a larger survey of intermediately active M dwarfs. GJ 4334 flared during the observation, starting with a rise in the flux of optical chromospheric emission lines, followed by the rapid rise and decay of multiple far-ultraviolet emission lines formed in the transition region, followed by the slow decay of the optical lines. We find significant broadening and asymmetries in the optical emission lines that are potentially from bulk plasma motion, a postflare elevated flux in both the optical and far-ultraviolet, and trends in the rise and decay timescales of the Balmer series such that higher-order lines rise earlier and decay faster than lower-order lines. The equivalent durations of the flare in individual lines range from 800 to 3 × 10⁴ s, mapping to flare energies of 1 × 10²⁸-3 × 10²⁹ erg for each line. To contextualize GJ 4334's flare behavior, we measure and compare its optical flare frequency distribution with TESS to EV Lacertae, a similar mass but faster rotating M dwarf, and find that GJ 4334 has an excess of large flares relative to the power law established by the majority of its smaller flares. This data set is a rare opportunity to characterize flares near a critical transition in stellar magnetic activity.