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

Abstract

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∞E ^-α ) with α∼-1.5 for 10³³<E<10³⁶ erg which is consistent with the previous results. The average occurrence rate of superflares with the energy of 10³³ erg which is equivalent to X100 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 (τ) increases with the flare energy (E) as τ∞E ^{0.39 ± 0.03}. This can be explained if we assume the time scale of flares is determined by the Alfvén time.