Statistical properties of superflares on solar-type stars based on 1-min cadence data

We searched for superflares on solar-type stars using Kepler data with 1-min sampling in order to detect superflares with a short duration. We found 187 superflares on 23 solar-type stars whose bolometric energy ranges from the order of 1032 to 1036 erg. Some superflares show multiple peaks with the peak separation of the order of 100 to 1,000 s which is comparable to the periods of quasi-periodic pulsations in solar and stellar flares. Using these new data combined with the results from the data with 30-min sampling, we found that the occurrence frequency (dN/dE) of superflares as a function of flare energy (E) shows the power-law distribution (dN/dE alpha E-alpha) with alpha similar to -1.5 for 10(33) < E < 10(36) erg which is consistent with the previous results. The average occurrence rate of superflares with the energy of 1033 erg which is equivalent to X 100 solar flares is about once in 500 to 600 years. The upper limit of energy released by superflares is basically comparable to a fraction of the magnetic energy stored near starspots which is estimated from the photometry. We also found that the duration of superflares (tau) increases with the flare energy (E) as tau alpha E-0.39 +/- 0.03. This can be explained if we assume the time scale of flares is determined by the Alfven time.