K2 Ultracool Dwarfs Survey - VI. White light superflares observed on an L5 dwarf and flare rates of L dwarfs

Kepler K2 long cadence data are used to study white light flares in a sample of 45 L dwarfs. We identified 11 flares on 9 L dwarfs with equivalent durations of (1.3-198) h and total (UV/optical/IR) energies of >= 0.9 x 10(32) erg. Two superflares with energies of >10(33) erg were detected on an L5 dwarf (VVV BD001): this is the coolest object so far on which flares have been identified. The larger superflare on this L5 dwarf has an energy of 4.6 x 10(34) erg and an amplitude of >300 times the photospheric level: so far, this is the largest amplitude flare detected by the Kepler/K2 mission. The next coolest star on which we identified a flare was an L2 dwarf: 2MASS J08585891+1804463. Combining the energies of all the flares which we have identified on 9 L dwarfs with the total observation time which was dedicated by Kepler to all 45 L dwarfs, we construct a composite flare frequency distribution (FFD). The FFD slope is quite shallow (-0.51 +/- 0.17), consistent with earlier results reported by Paudel et al. for one particular L0 dwarf, for which the FFD slope was found to be -0.34. Using the composite FFD, we predict that, in early- and mid-L dwarfs, a superflare of energy 10(33) erg occurs every 2.4 yr and a superflare of energy 10(34) erg occurs every 7.9 yr. Analysis of our L dwarf flares suggests that magnetic fields of >= 0.13-1.3 kG are present on the stellar surface: such fields could suppress Type II radio bursts.