We searched for superflares on solar-type stars using the Kepler short-cadence ( 1-min sampling ) data in order to detect superflares with short duration .
We found 187 superflares on 23 solar-type stars whose bolometric energy ranges from the order of 10 ^ { 32 } erg to 10 ^ { 36 } erg .
Using these new data combined with the results from the data with 30-min sampling , we found the occurrence frequency ( dN / dE ) of superflares as a function of flare energy ( E ) shows the power-law distribution ( dN / dE \propto E ^ { - \alpha } ) with \alpha = 1.5 for 10 ^ { 33 } < E < 10 ^ { 36 } erg .
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 amplitude of brightness variations .
We also found that the duration of superflares ( \tau ) increases with the flare energy ( E ) as \tau \propto E ^ { 0.39 \pm 0.03 } .
This can be explained if we assume the time-scale of flares is determined by the Alfv \acute { e } n time .